diff --git a/mjx/mujoco/mjx/_src/collision_driver.py b/mjx/mujoco/mjx/_src/collision_driver.py
index 3371a674..70d3a59b 100644
--- a/mjx/mujoco/mjx/_src/collision_driver.py
+++ b/mjx/mujoco/mjx/_src/collision_driver.py
@@ -349,8 +349,8 @@ def make_condim(
     m: Union[Model, mujoco.MjModel], impl: Impl = Impl.JAX
 ) -> np.ndarray:
   """Returns the dims of the contacts for a Model."""
-  if impl not in (Impl.JAX, Impl.C):
-    raise ValueError('make_condim only supports JAX and C backends.')
+  if impl != Impl.JAX:
+    raise ValueError('make_condim only supports JAX backend.')
 
   if isinstance(m, mujoco.MjModel):
     sdf_initpoints = m.opt.sdf_initpoints
@@ -389,8 +389,6 @@ def make_condim(
       func = _COLLISION_FUNC.get(k.types, None)
       if func is not None:
         ncon = func.ncon  # pytype: disable=attribute-error
-      elif impl == Impl.C:
-        ncon = _MAX_NCON
       else:
         raise ValueError(
             f'Collision function not found for geom types {k.types[0]},',
diff --git a/mjx/mujoco/mjx/_src/forward_test.py b/mjx/mujoco/mjx/_src/forward_test.py
index f90fae1a..ba200b72 100644
--- a/mjx/mujoco/mjx/_src/forward_test.py
+++ b/mjx/mujoco/mjx/_src/forward_test.py
@@ -170,6 +170,7 @@ class ForwardTest(absltest.TestCase):
     np.testing.assert_allclose(dx.qvel, 1 + m.opt.timestep)
 
 
+
 class ActuatorTest(parameterized.TestCase):
 
   @parameterized.parameters(
diff --git a/mjx/mujoco/mjx/_src/io.py b/mjx/mujoco/mjx/_src/io.py
index 253a62a5..54e40926 100644
--- a/mjx/mujoco/mjx/_src/io.py
+++ b/mjx/mujoco/mjx/_src/io.py
@@ -90,7 +90,7 @@ def _resolve_device(
     logging.debug('Picking default device: %s.', device_0)
     return device_0
 
-  if impl == types.Impl.C or impl == types.Impl.CPP:
+  if impl == types.Impl.CPP:
     cpu_0 = jax.devices('cpu')[0]
     logging.debug('Picking default device: %s', cpu_0)
     return cpu_0
@@ -128,7 +128,7 @@ def _check_impl_device_compatibility(
     _check_warp_installed()
 
   is_cpu_device = device.platform == 'cpu'
-  if impl == types.Impl.C or impl == types.Impl.CPP:
+  if impl == types.Impl.CPP:
     if not is_cpu_device:
       raise AssertionError(
           f'C implementation requires a CPU device, got {device}.'
@@ -248,7 +248,6 @@ def _put_option(
   fields['jacobian'] = types.JacobianType(o.jacobian)
 
   option_obj = {
-      types.Impl.C: types.OptionC,
       types.Impl.JAX: types.OptionJAX,
       types.Impl.WARP: mjxw.types.OptionWarp,
   }[impl]
@@ -266,8 +265,6 @@ def _put_option(
     impl_fields['has_fluid_params'] = has_fluid_params
     return types.Option(**fields, _impl=types.OptionJAX(**impl_fields))
 
-  if impl == types.Impl.C:
-    return types.Option(**fields, _impl=types.OptionC(**impl_fields))
 
   if impl == types.Impl.WARP:
     impl_fields = {k: _wp_to_np_type(v) for k, v in impl_fields.items()}
@@ -425,28 +422,6 @@ def _put_model_jax(
   return _strip_weak_type(model)
 
 
-def _put_model_c(
-    m: mujoco.MjModel,
-    device: Optional[jax.Device] = None,
-) -> types.Model:
-  """Puts mujoco.MjModel onto a device, resulting in mjx.Model."""
-  mj_field_names = {f.name for f in types.Model.fields() if f.name != '_impl'}
-  fields = {f: getattr(m, f) for f in mj_field_names}
-  fields['cam_mat0'] = fields['cam_mat0'].reshape((-1, 3, 3))
-  fields['opt'] = _put_option(m.opt, impl=types.Impl.C)
-  fields['stat'] = _put_statistic(m.stat, impl=types.Impl.C)
-
-  c_impl_keys = (
-      types.ModelC.__annotations__.keys() - types.Model.__annotations__.keys()
-  )
-  c_impl_dict = {k: getattr(m, k) for k in c_impl_keys}
-  c_impl_obj = types.ModelC(**{k: copy.copy(v) for k, v in c_impl_dict.items()})
-
-  model = types.Model(
-      **{k: copy.copy(v) for k, v in fields.items()}, _impl=c_impl_obj
-  )
-  model = jax.device_put(model, device=device)
-  return _strip_weak_type(model)
 
 
 def _put_model_warp(
@@ -506,8 +481,8 @@ def _put_model_cpp(
   mj_field_names = {f.name for f in types.Model.fields() if f.name != '_impl'}
   fields = {f: getattr(m, f) for f in mj_field_names}
   fields['cam_mat0'] = fields['cam_mat0'].reshape((-1, 3, 3))
-  fields['opt'] = _put_option(m.opt, impl=types.Impl.C)
-  fields['stat'] = _put_statistic(m.stat, impl=types.Impl.C)
+  fields['opt'] = _put_option(m.opt, impl=types.Impl.JAX)
+  fields['stat'] = _put_statistic(m.stat, impl=types.Impl.JAX)
 
   # get the pointer address
   # we use a 0-d array
@@ -562,8 +537,6 @@ def put_model(
   impl, device = _resolve_impl_and_device(impl, device)
   if impl == types.Impl.JAX:
     return _put_model_jax(m, device)
-  elif impl == types.Impl.C:
-    return _put_model_c(m, device)
   elif impl == types.Impl.WARP:
     _check_warp_installed()
     graph_mode = graph_mode or getattr(mjxw.types.GraphMode, 'WARP')
@@ -740,125 +713,6 @@ def _make_data_jax(
   return d
 
 
-def _make_data_c(
-    m: Union[types.Model, mujoco.MjModel],
-    device: Optional[jax.Device] = None,
-) -> types.Data:
-  """Allocate and initialize Data for the C implementation."""
-  # TODO(stunya): The C implementation should not use static dimensions, and
-  # the backend implementation details should be kept hidden from JAX
-  # altogether.
-  dim = collision_driver.make_condim(m, impl=types.Impl.C)
-  efc_type = constraint.make_efc_type(m, dim)
-  efc_address = constraint.make_efc_address(m, dim, efc_type)
-  ne, nf, nl, nc = constraint.counts(efc_type)
-  ncon, nefc = dim.size, ne + nf + nl + nc
-
-  float_ = jp.zeros(1, float).dtype
-  int_ = jp.zeros(1, int).dtype
-  # TODO(stunya): remove the JAX contact from C data.
-  contact = _make_data_contact_jax(dim, efc_address)
-
-  def get(m, name: str):
-    return getattr(m._impl, name) if hasattr(m, '_impl') else getattr(m, name)  # pylint: disable=protected-access
-
-  nflexvert = get(m, 'nflexvert')
-  nflexedge = get(m, 'nflexedge')
-  nflexelem = get(m, 'nflexelem')
-  nbvh = get(m, 'nbvh')
-  nbvhdynamic = get(m, 'nbvhdynamic')
-  zero_impl_fields = {
-      'solver_niter': (int_,),
-      'cinert': (m.nbody, 10, float_),
-      'light_xpos': (m.nlight, 3, float_),
-      'light_xdir': (m.nlight, 3, float_),
-      'flexvert_xpos': (nflexvert, 3, float_),
-      'flexelem_aabb': (nflexelem, 6, float_),
-      'flexedge_J': (m.nJfe, float_),
-      'flexedge_length': (nflexedge, float_),
-      'ten_J_rownnz': (m.ntendon, np.int32),
-      'ten_J_rowadr': (m.ntendon, np.int32),
-      'ten_J_colind': (m.nJten, np.int32),
-      'ten_J': (m.nJten, float_),
-      'ten_wrapadr': (m.ntendon, np.int32),
-      'ten_wrapnum': (m.ntendon, np.int32),
-      'wrap_obj': (m.nwrap, 2, np.int32),
-      'wrap_xpos': (m.nwrap, 6, float_),
-      'moment_rownnz': (m.nu, np.int32),
-      'moment_rowadr': (m.nu, np.int32),
-      'moment_colind': (m.nJmom, np.int32),
-      'actuator_moment': (m.nJmom, float_),
-      'bvh_aabb_dyn': (nbvhdynamic, 6, float_),
-      'bvh_active': (nbvh, np.uint8),
-      'tree_asleep': (m.ntree, int_),
-      'tree_awake': (m.ntree, int_),
-      'body_awake': (m.nbody, int_),
-      'body_awake_ind': (m.nbody, int_),
-      'parent_awake_ind': (m.nbody, int_),
-      'dof_awake_ind': (m.nv, int_),
-      'tree_island': (m.ntree, int_),
-      'map_itree2tree': (m.ntree, int_),
-      'flexedge_velocity': (nflexedge, float_),
-      'crb': (m.nbody, 10, float_),
-      'qM': (m.nM, float_),
-      'M': (m.nC, float_),
-      'qLD': (m.nC, float_),
-      'qH': (m.nC, float_),
-      'qHDiagInv': (m.nv, float_),
-      'qLDiagInv': (m.nv, float_),
-      'ten_velocity': (m.ntendon, float_),
-      'actuator_velocity': (m.nu, float_),
-      'plugin_data': (get(m, 'nplugin'), np.uint64),
-      'qDeriv': (m.nD, float_),
-      'qLU': (m.nD, float_),
-      'qfrc_spring': (m.nv, float_),
-      'qfrc_damper': (m.nv, float_),
-      'cacc': (m.nbody, 6, float_),
-      'cfrc_int': (m.nbody, 6, float_),
-      'cfrc_ext': (m.nbody, 6, float_),
-      'subtree_linvel': (m.nbody, 3, float_),
-      'subtree_angmom': (m.nbody, 3, float_),
-      'efc_J': (nefc, m.nv, float_),
-      'efc_pos': (nefc, float_),
-      'efc_margin': (nefc, float_),
-      'efc_frictionloss': (nefc, float_),
-      'efc_D': (nefc, float_),
-      'efc_aref': (nefc, float_),
-      'efc_force': (nefc, float_),
-  }
-  zero_impl_fields = {
-      k: np.zeros(v[:-1], dtype=v[-1]) for k, v in zero_impl_fields.items()
-  }
-  impl = types.DataC(
-      ne=ne,
-      nf=nf,
-      nl=nl,
-      nefc=nefc,
-      ncon=ncon,
-      contact=contact,
-      efc_type=efc_type,
-      **zero_impl_fields,
-  )
-
-  d = types.Data(
-      qpos=jp.array(m.qpos0, dtype=float_),
-      eq_active=m.eq_active0,
-      _impl=impl,
-      **_make_data_public_fields(m),
-  )
-
-  if m.nmocap:
-    # Set mocap_pos/quat = body_pos/quat for mocap bodies as done in C MuJoCo.
-    body_mask = m.body_mocapid >= 0
-    body_pos = m.body_pos[body_mask]
-    body_quat = m.body_quat[body_mask]
-    d = d.replace(
-        mocap_pos=body_pos[m.body_mocapid[body_mask]],
-        mocap_quat=body_quat[m.body_mocapid[body_mask]],
-    )
-
-  d = jax.device_put(d, device=device)
-  return d
 
 
 def _get_nested_attr(obj: Any, attr_name: str, split: str) -> Any:
@@ -1024,8 +878,6 @@ def make_data(
 
   if impl == types.Impl.JAX:
     return _make_data_jax(m, device)
-  elif impl == types.Impl.C:
-    return _make_data_c(m, device)
   elif impl == types.Impl.CPP:
     return _make_data_cpp(m, device, keepalive_refs=keepalive_refs)
   elif impl == types.Impl.WARP:
@@ -1226,111 +1078,6 @@ def _put_data_jax(
   return _strip_weak_type(data)
 
 
-def _put_data_c(
-    m: mujoco.MjModel, d: mujoco.MjData, device: Optional[jax.Device] = None
-) -> types.Data:
-  """Puts mujoco.MjData onto a device, resulting in mjx.Data."""
-  # TODO(stunya): ncon, nefc should potentially be jax.Array, and contact/efc
-  # should not be materialized in JAX.
-  dim = collision_driver.make_condim(m, impl=types.Impl.C)
-  efc_type = constraint.make_efc_type(m, dim)
-  efc_address = constraint.make_efc_address(m, dim, efc_type)
-  ne, nf, nl, nc = constraint.counts(efc_type)
-  ncon, nefc = dim.size, ne + nf + nl + nc
-
-  # TODO(stunya): remove this check.
-  for d_val, val, name in (
-      (d.ncon, ncon, 'ncon'),
-      (d.ne, ne, 'ne'),
-      (d.nf, nf, 'nf'),
-      (d.nl, nl, 'nl'),
-      (d.nefc, nefc, 'nefc'),
-  ):
-    if d_val > val:
-      raise ValueError(f'd.{name} too high, d.{name} = {d_val}, model = {val}')
-
-  fields = _put_data_public_fields(d)
-
-  # Implementation specific fields.
-  impl_fields = {
-      f.name: getattr(d, f.name)
-      for f in types.DataC.fields()
-      if hasattr(d, f.name)
-  }
-  for f in types.DataC.fields():
-    if not hasattr(d, f.name) and hasattr(m, f.name):
-      impl_fields[f.name] = getattr(m, f.name)
-
-  # TODO(stunya): support islanding via C impl.
-  impl_fields['solver_niter'] = impl_fields['solver_niter'][0]
-
-  # TODO(stunya): remove reliance on JAX _put_contact.
-  contact, contact_map = _put_contact(d.contact, dim, efc_address)
-
-  # TODO(stunya): remove reliance on dense efc_J.
-  if mujoco.mj_isSparse(m):
-    efc_j = np.zeros((d.efc_J_rownnz.shape[0], m.nv))
-    mujoco.mju_sparse2dense(
-        efc_j,
-        impl_fields['efc_J'],
-        d.efc_J_rownnz,
-        d.efc_J_rowadr,
-        d.efc_J_colind,
-    )
-    impl_fields['efc_J'] = efc_j
-  else:
-    impl_fields['efc_J'] = impl_fields['efc_J'].reshape(
-        (-1 if m.nv else 0, m.nv)
-    )
-
-  # move efc rows to their correct offsets
-  for fname in (
-      'efc_J',
-      'efc_pos',
-      'efc_margin',
-      'efc_frictionloss',
-      'efc_D',
-      'efc_aref',
-      'efc_force',
-  ):
-    value = np.zeros((nefc, m.nv)) if fname == 'efc_J' else np.zeros(nefc)
-    for i in range(3):
-      value_beg = sum([ne, nf][:i])
-      d_beg = sum([d.ne, d.nf][:i])
-      size = [d.ne, d.nf, d.nl][i]
-      value[value_beg : value_beg + size] = impl_fields[fname][
-          d_beg : d_beg + size
-      ]
-
-    # for nc, we may reorder contacts so they match MJX order: group by dim
-    for id_to, id_from in enumerate(contact_map):
-      if id_from == -1:
-        continue
-      num_rows = dim[id_to]
-      if num_rows > 1 and m.opt.cone == mujoco.mjtCone.mjCONE_PYRAMIDAL:
-        num_rows = (num_rows - 1) * 2
-      efc_i, efc_o = d.contact.efc_address[id_from], efc_address[id_to]
-      if efc_i == -1:
-        continue
-      value[efc_o : efc_o + num_rows] = impl_fields[fname][
-          efc_i : efc_i + num_rows
-      ]
-
-    impl_fields[fname] = value
-
-  impl_fields['contact'] = contact
-  impl_fields.update(
-      ne=ne, nf=nf, nl=nl, nefc=nefc, ncon=ncon, efc_type=efc_type
-  )
-
-  # copy because device_put is async:
-  data_jax = types.DataC(**{k: copy.copy(v) for k, v in impl_fields.items()})
-  data = types.Data(
-      **{k: copy.copy(v) for k, v in fields.items()}, _impl=data_jax
-  )
-
-  data = jax.device_put(data, device=device)
-  return _strip_weak_type(data)
 
 
 # TODO(josechenf): Iterate on the keepalive implementation to make it easier to
@@ -1470,8 +1217,6 @@ def put_data(
   impl, device = _resolve_impl_and_device(impl, device)
   if impl == types.Impl.JAX:
     return _put_data_jax(m, d, device)
-  elif impl == types.Impl.C:
-    return _put_data_c(m, d, device)
   elif impl == types.Impl.CPP:
     return _put_data_cpp(
         m,
@@ -1613,8 +1358,6 @@ def _get_data_into(
 
     if d.impl == types.Impl.JAX:
       all_fields = types.Data.fields() + types.DataJAX.fields()
-    elif d.impl == types.Impl.C:
-      all_fields = types.Data.fields() + types.DataC.fields()
     else:
       raise NotImplementedError(
           f'get_data_into for implementation "{d.impl}" not implemented yet.'
@@ -1814,7 +1557,7 @@ def get_data_into(
 
   d = jax.device_get(d)
 
-  if d.impl in (types.Impl.JAX, types.Impl.C):
+  if d.impl == types.Impl.JAX:
     # TODO(stunya): Split out _get_data_into once codepaths diverge enough.
     return _get_data_into(result, m, d)
 
diff --git a/mjx/mujoco/mjx/_src/io_test.py b/mjx/mujoco/mjx/_src/io_test.py
index 9910e9b5..dfcb541d 100644
--- a/mjx/mujoco/mjx/_src/io_test.py
+++ b/mjx/mujoco/mjx/_src/io_test.py
@@ -136,7 +136,7 @@ class ModelIOTest(parameterized.TestCase):
 
   @parameterized.product(
       xml=(_MULTIPLE_CONVEX_OBJECTS, _MULTIPLE_CONSTRAINTS),
-      impl=('jax', 'c', 'warp', 'cpp'),
+      impl=('jax', 'warp', 'cpp'),
   )
   @mock.patch.dict(os.environ, {'MJX_GPU_DEFAULT_WARP': 'true'})
   def test_put_model(self, xml, impl):
@@ -171,11 +171,7 @@ class ModelIOTest(parameterized.TestCase):
     if impl == 'jax':
       # fields restricted to MuJoCo should not be populated
       self.assertFalse(hasattr(mx, 'bvh_aabb'))
-    elif impl == 'c':
-      # Options specific to C are populated.
-      self.assertEqual(mx.opt._impl.noslip_iterations, m.opt.noslip_iterations)
-      # Fields private to C backend impl are populated.
-      self.assertTrue(hasattr(mx._impl, 'bvh_aabb'))
+
     elif impl == 'warp':
       # Options specific to Warp are populated.
       self.assertTrue(hasattr(mx.opt._impl, 'ls_parallel'))
@@ -353,8 +349,7 @@ class ModelIOTest(parameterized.TestCase):
     expected = graph_mode or mjxw_types.GraphMode.WARP
     self.assertEqual(mx.opt._impl.graph_mode, expected)
 
-  @parameterized.parameters('c', 'jax')
-  def test_unsupported_contact_types(self, impl):
+  def test_unsupported_contact_types(self):
     """Tests that unsupported contact types raise an error."""
     m = mujoco.MjModel.from_xml_string("""
       <mujoco>
@@ -374,11 +369,8 @@ class ModelIOTest(parameterized.TestCase):
       </mujoco>
     """)
 
-    if impl == 'jax':
-      with self.assertRaises(ValueError):
-        mjx.make_data(m, impl=impl)
-    if impl == 'c':
-      mjx.make_data(m, impl=impl)
+    with self.assertRaises(ValueError):
+      mjx.make_data(m, impl='jax')
 
 
 class DataIOTest(parameterized.TestCase):
@@ -390,7 +382,7 @@ class DataIOTest(parameterized.TestCase):
       self.tempdir = tempfile.TemporaryDirectory()
       wp.config.kernel_cache_dir = self.tempdir.name
 
-  @parameterized.parameters('jax', 'c', 'cpp')
+  @parameterized.parameters('jax', 'cpp')
   def test_make_data(self, impl: str):
     """Test that make_data returns the correct shapes."""
     m = mujoco.MjModel.from_xml_string(_MULTIPLE_CONVEX_OBJECTS)
@@ -437,8 +429,7 @@ class DataIOTest(parameterized.TestCase):
     self.assertEqual(d._impl.crb.shape, (nbody, 10))
     if impl == 'jax':
       self.assertEqual(d._impl.actuator_moment.shape, (1, nv))
-    elif impl == 'c':
-      self.assertEqual(d._impl.actuator_moment.shape, (m.nJmom,))
+
     self.assertEqual(d._impl.contact.dist.shape, (ncon,))
     self.assertEqual(d._impl.contact.pos.shape, (ncon, 3))
     self.assertEqual(d._impl.contact.frame.shape, (ncon, 3, 3))
@@ -466,10 +457,7 @@ class DataIOTest(parameterized.TestCase):
       self.assertEqual(d._impl.qM.shape, (nv, nv))
       self.assertEqual(d._impl.qLD.shape, (nv, nv))
       self.assertEqual(d._impl.qLDiagInv.shape, (0,))
-    elif impl == 'c':
-      self.assertEqual(d._impl.qM.shape, (nm,))
-      self.assertEqual(d._impl.qLD.shape, (nm,))
-      self.assertEqual(d._impl.qLDiagInv.shape, (nv,))
+
 
     # test sparse
     m.opt.jacobian = mujoco.mjtJacobian.mjJAC_SPARSE
@@ -478,10 +466,7 @@ class DataIOTest(parameterized.TestCase):
     self.assertEqual(d._impl.qLD.shape, (nm,))
     self.assertEqual(d._impl.qLDiagInv.shape, (nv,))
 
-    if impl == 'c':
-      # check C specific fields
-      self.assertEqual(d._impl.light_xpos.shape, (m.nlight, 3))
-      self.assertEqual(d._impl.bvh_active.shape, (m.nbvh,))
+
 
   @mock.patch.dict(os.environ, {'MJX_GPU_DEFAULT_WARP': 'true'})
   def test_make_data_warp(self):
@@ -494,7 +479,7 @@ class DataIOTest(parameterized.TestCase):
     self.assertEqual(d._impl.contact__dist.shape[0], 9)
     self.assertEqual(d._impl.efc__pos.shape[0], 23)
 
-  @parameterized.parameters('jax', 'c', 'cpp', 'warp')
+  @parameterized.parameters('jax', 'cpp', 'warp')
   def test_put_data(self, impl: str):
     """Test that put_data puts the correct data for dense and sparse."""
     if impl == 'warp':
@@ -537,10 +522,7 @@ class DataIOTest(parameterized.TestCase):
       qm = np.zeros((m.nv, m.nv), dtype=np.float64)
       mujoco.mj_fullM(m, qm, d.qM)
       np.testing.assert_allclose(qm, mjx.full_m(mjx.put_model(m), dx))
-    elif impl == 'c':
-      np.testing.assert_allclose(dx._impl.qM, d.qM)
-      np.testing.assert_allclose(dx._impl.qLD, d.qLD)
-      np.testing.assert_allclose(dx._impl.qLDiagInv, d.qLDiagInv)
+
     elif impl == 'cpp':
       self.assertTrue(hasattr(dx._impl, 'pointer_lo'))
       self.assertTrue(hasattr(dx._impl, 'pointer_hi'))
@@ -616,8 +598,7 @@ class DataIOTest(parameterized.TestCase):
       qm = np.zeros((m.nv, m.nv))
       mujoco.mj_fullM(m, qm, d.qM)
       np.testing.assert_allclose(dx_from_dense._impl.qM, qm, atol=1e-8)
-    elif impl == 'c':
-      np.testing.assert_allclose(dx_from_dense._impl.qM, d.qM, atol=1e-8)
+
 
   def test_put_data_warp_ndim(self):
     """Tests that put_data produces expected dimensions for Warp fields."""
@@ -645,7 +626,7 @@ class DataIOTest(parameterized.TestCase):
     _ = jax.tree.map_with_path(check_ndim, dx)
 
   @parameterized.parameters(
-      ('jax', False), ('jax', True), ('c', False), ('c', True)
+      ('jax', False), ('jax', True)
   )
   def test_get_data(self, impl: str, sparse: bool):
     """Test that get_data makes correct MjData."""
@@ -710,9 +691,6 @@ class DataIOTest(parameterized.TestCase):
     np.testing.assert_allclose(d_2.efc_aref, d.efc_aref)
     np.testing.assert_allclose(d_2.contact.efc_address, d.contact.efc_address)
 
-    if impl == 'c':
-      # check fields specific to the C implementation
-      np.testing.assert_allclose(d_2.bvh_active, d.bvh_active)
 
   def test_get_data_simplebody(self):
     """Test that get_data works with simple bodies where nC < nM."""
@@ -744,7 +722,7 @@ class DataIOTest(parameterized.TestCase):
     dx = mjx.put_data(m, d)
     mjx.get_data(m, dx)
 
-  @parameterized.parameters('jax', 'c')
+  @parameterized.parameters(('jax',))
   def test_get_data_batched(self, impl):
     """Test that get_data makes correct List[MjData] for batched Data."""
 
@@ -761,7 +739,7 @@ class DataIOTest(parameterized.TestCase):
     self.assertEqual(ds[0].ncon, 1)
     self.assertEqual(ds[1].ncon, 0)
 
-  @parameterized.parameters('jax', 'c', 'cpp')
+  @parameterized.parameters('jax', 'cpp')
   def test_get_data_into(self, impl):
     """Test that get_data_into correctly populates an MjData."""
 
@@ -801,7 +779,7 @@ class DataIOTest(parameterized.TestCase):
     dx = mjx.make_data(m, impl='warp')
     mjx.get_data_into(d, mx, dx)
 
-  @parameterized.parameters('jax', 'c')
+  @parameterized.parameters(('jax',))
   def test_get_data_into_wrong_shape(self, impl):
     """Tests that get_data_into throwsif input and output shapes don't match."""
 
@@ -814,7 +792,7 @@ class DataIOTest(parameterized.TestCase):
     with self.assertRaisesRegex(ValueError, r'Input field.*has shape.*'):
       mjx.get_data_into(d_2, m, dx)
 
-  @parameterized.parameters('jax', 'c')
+  @parameterized.parameters(('jax',))
   def test_make_matches_put(self, impl):
     """Test that make_data produces a pytree that matches put_data."""
     m = mujoco.MjModel.from_xml_string(_MULTIPLE_CONSTRAINTS)
@@ -960,11 +938,11 @@ _DEVICE_TEST_CASES = [
     ('gpu-notnvidia', 'warp', ('cpu', 'error')),
     ('gpu-nvidia', 'warp', ('gpu', Impl.WARP)),
     ('tpu', 'warp', ('tpu', 'error')),
-    # C backend specified.
-    ('cpu', 'c', ('cpu', Impl.C)),
-    ('gpu-notnvidia', 'c', ('cpu', 'error')),
-    ('gpu-nvidia', 'c', ('cpu', 'error')),
-    ('tpu', 'c', ('tpu', 'error')),
+    # CPP backend specified.
+    ('cpu', 'cpp', ('cpu', Impl.CPP)),
+    ('gpu-notnvidia', 'cpp', ('cpu', 'error')),
+    ('gpu-nvidia', 'cpp', ('cpu', 'error')),
+    ('tpu', 'cpp', ('tpu', 'error')),
 ]
 
 # Test cases for `_resolve_impl_and_device` where the user does NOT
@@ -988,11 +966,11 @@ _DEFAULT_DEVICE_TEST_CASES = [
     ('gpu-notnvidia', 'warp', ('cpu', 'error')),
     ('gpu-nvidia', 'warp', ('gpu', Impl.WARP)),
     ('tpu', 'warp', ('tpu', 'error')),
-    # C backend impl specified, CPU should always be available.
-    ('cpu', 'c', ('cpu', Impl.C)),
-    ('gpu-notnvidia', 'c', ('cpu', Impl.C)),
-    ('gpu-nvidia', 'c', ('cpu', Impl.C)),
-    ('tpu', 'c', ('cpu', Impl.C)),
+    # CPP backend impl specified, CPU should always be available.
+    ('cpu', 'cpp', ('cpu', Impl.CPP)),
+    ('gpu-notnvidia', 'cpp', ('cpu', Impl.CPP)),
+    ('gpu-nvidia', 'cpp', ('cpu', Impl.CPP)),
+    ('tpu', 'cpp', ('cpu', Impl.CPP)),
 ]
 
 
diff --git a/mjx/mujoco/mjx/_src/types.py b/mjx/mujoco/mjx/_src/types.py
index 898c844a..5b8ad301 100644
--- a/mjx/mujoco/mjx/_src/types.py
+++ b/mjx/mujoco/mjx/_src/types.py
@@ -29,8 +29,8 @@ import numpy as np
 class Impl(enum.Enum):
   """Implementation to use."""
 
-  C = 'c'
   CPP = 'cpp'
+  C = 'cpp'  # alias C -> CPP
   JAX = 'jax'
   WARP = 'warp'
 
@@ -490,22 +490,6 @@ class OptionJAX(PyTreeNode):
   has_fluid_params: bool
 
 
-class OptionC(PyTreeNode):
-  """C-specific option."""
-
-  o_margin: jax.Array
-  o_solref: jax.Array
-  o_solimp: jax.Array
-  o_friction: jax.Array
-  disableactuator: int
-  sdf_initpoints: int
-  has_fluid_params: bool
-  noslip_tolerance: jax.Array
-  ccd_tolerance: jax.Array
-  sleep_tolerance: jax.Array
-  noslip_iterations: int
-  ccd_iterations: int
-  sdf_iterations: int
 
 
 class Option(PyTreeNode):
@@ -528,7 +512,7 @@ class Option(PyTreeNode):
   integrator: IntegratorType
   solver: SolverType
   timestep: jax.Array
-  _impl: Union[OptionJAX, OptionC, mjxw_types.OptionWarp]
+  _impl: Union[OptionJAX, mjxw_types.OptionWarp]
 
 
 class ModelCPP(PyTreeNode):
@@ -549,121 +533,6 @@ class DataCPP(PyTreeNode):
   pointer_hi: jax.Array
 
 
-class ModelC(PyTreeNode):
-  """CPU-specific model data."""
-
-  nbvh: jax.Array
-  nbvhstatic: jax.Array
-  nbvhdynamic: jax.Array
-  ntree: jax.Array
-  nflex: jax.Array
-  nflexvert: jax.Array
-  nflexedge: jax.Array
-  nflexelem: jax.Array
-  nflexelemdata: jax.Array
-  nflexshelldata: jax.Array
-  nflexevpair: jax.Array
-  nflextexcoord: jax.Array
-  nplugin: jax.Array
-  narena: jax.Array
-  body_bvhadr: jax.Array
-  body_bvhnum: jax.Array
-  bvh_child: jax.Array
-  bvh_nodeid: jax.Array
-  bvh_aabb: jax.Array
-  oct_child: jax.Array
-  oct_aabb: jax.Array
-  oct_coeff: jax.Array
-  dof_length: jax.Array
-  tree_bodyadr: jax.Array
-  tree_bodynum: jax.Array
-  tree_dofadr: jax.Array
-  tree_dofnum: jax.Array
-  tree_sleep_policy: jax.Array
-  geom_plugin: jax.Array
-  light_bodyid: jax.Array
-  light_targetbodyid: jax.Array
-  flex_contype: jax.Array
-  flex_conaffinity: jax.Array
-  flex_condim: jax.Array
-  flex_priority: jax.Array
-  flex_solmix: jax.Array
-  flex_solref: jax.Array
-  flex_solimp: jax.Array
-  flex_friction: jax.Array
-  flex_margin: jax.Array
-  flex_gap: jax.Array
-  flex_internal: jax.Array
-  flex_selfcollide: jax.Array
-  flex_activelayers: jax.Array
-  flex_passive: jax.Array
-  flex_dim: jax.Array
-  flex_vertadr: jax.Array
-  flex_vertnum: jax.Array
-  flex_edgeadr: jax.Array
-  flex_edgenum: jax.Array
-  flex_elemadr: jax.Array
-  flex_elemnum: jax.Array
-  flex_elemdataadr: jax.Array
-  flex_evpairadr: jax.Array
-  flex_evpairnum: jax.Array
-  flex_vertbodyid: jax.Array
-  flex_edge: jax.Array
-  flex_elem: jax.Array
-  flex_elemlayer: jax.Array
-  flex_evpair: jax.Array
-  flex_vert: jax.Array
-  flexedge_length0: jax.Array
-  flexedge_invweight0: jax.Array
-  flex_radius: jax.Array
-  flex_edgestiffness: jax.Array
-  flex_edgedamping: jax.Array
-  flex_edgeequality: jax.Array
-  flex_rigid: jax.Array
-  flexedge_rigid: jax.Array
-  flex_centered: jax.Array
-  flex_bvhadr: jax.Array
-  flex_bvhnum: jax.Array
-  flexedge_J_rownnz: jax.Array
-  flexedge_J_rowadr: jax.Array
-  flexedge_J_colind: jax.Array
-  mesh_polynum: jax.Array
-  mesh_polyadr: jax.Array
-  mesh_polynormal: jax.Array
-  mesh_polyvertadr: jax.Array
-  mesh_polyvertnum: jax.Array
-  mesh_polyvert: jax.Array
-  mesh_polymapadr: jax.Array
-  mesh_polymapnum: jax.Array
-  mesh_polymap: jax.Array
-  tendon_treenum: jax.Array
-  tendon_treeid: jax.Array
-  actuator_plugin: jax.Array
-  actuator_history: jax.Array
-  actuator_historyadr: jax.Array
-  actuator_delay: jax.Array
-  sensor_plugin: jax.Array
-  sensor_history: jax.Array
-  sensor_historyadr: jax.Array
-  sensor_delay: jax.Array
-  sensor_interval: jax.Array
-  plugin: jax.Array
-  plugin_stateadr: jax.Array
-  B_rownnz: jax.Array  # pylint:disable=invalid-name
-  B_rowadr: jax.Array  # pylint:disable=invalid-name
-  B_colind: jax.Array  # pylint:disable=invalid-name
-  M_rownnz: jax.Array  # pylint:disable=invalid-name
-  M_rowadr: jax.Array  # pylint:disable=invalid-name
-  M_colind: jax.Array  # pylint:disable=invalid-name
-  mapM2M: jax.Array  # pylint:disable=invalid-name
-  D_rownnz: jax.Array  # pylint:disable=invalid-name
-  D_rowadr: jax.Array  # pylint:disable=invalid-name
-  D_diag: jax.Array  # pylint:disable=invalid-name
-  D_colind: jax.Array  # pylint:disable=invalid-name
-  mapM2D: jax.Array  # pylint:disable=invalid-name
-  mapD2M: jax.Array  # pylint:disable=invalid-name
-
-
 class ModelJAX(PyTreeNode):
   """JAX-specific model data."""
 
@@ -1025,12 +894,11 @@ class Model(PyTreeNode):
   names: bytes
   signature: np.uint64
   _sizes: jax.Array
-  _impl: Union[ModelC, ModelJAX, mjxw_types.ModelWarp]
+  _impl: Union[ModelJAX, mjxw_types.ModelWarp]
 
   @property
   def impl(self) -> Impl:
     return {
-        ModelC: Impl.C,
         ModelCPP: Impl.CPP,
         ModelJAX: Impl.JAX,
         mjxw_types.ModelWarp: Impl.WARP,
@@ -1094,82 +962,6 @@ class Contact(PyTreeNode):
   efc_address: np.ndarray
 
 
-class DataC(PyTreeNode):
-  """C-specific data."""
-
-  # constant sizes:
-  # TODO(stunya): make these sizes jax.Array?
-  ncon: int
-  ne: int
-  nf: int
-  nl: int
-  nefc: int
-  # TODO(stunya): remove most of these fields
-  solver_niter: jax.Array
-  tree_asleep: jax.Array
-  plugin_data: jax.Array
-  light_xpos: jax.Array
-  light_xdir: jax.Array
-  cinert: jax.Array
-  flexvert_xpos: jax.Array
-  flexelem_aabb: jax.Array
-  flexedge_J: jax.Array  # pylint:disable=invalid-name
-  flexedge_length: jax.Array
-  bvh_aabb_dyn: jax.Array
-  ten_wrapadr: jax.Array
-  ten_wrapnum: jax.Array
-  ten_J_rownnz: jax.Array  # pylint:disable=invalid-name
-  ten_J_rowadr: jax.Array  # pylint:disable=invalid-name
-  ten_J_colind: jax.Array  # pylint:disable=invalid-name
-  ten_J: jax.Array  # pylint:disable=invalid-name
-  wrap_obj: jax.Array
-  wrap_xpos: jax.Array
-  moment_rownnz: jax.Array  # pylint:disable=invalid-name
-  moment_rowadr: jax.Array  # pylint:disable=invalid-name
-  moment_colind: jax.Array  # pylint:disable=invalid-name
-  actuator_moment: jax.Array
-  crb: jax.Array
-  qM: jax.Array  # pylint:disable=invalid-name
-  M: jax.Array  # pylint:disable=invalid-name
-  qLD: jax.Array  # pylint:disable=invalid-name
-  qLDiagInv: jax.Array  # pylint:disable=invalid-name
-  bvh_active: jax.Array
-  tree_awake: jax.Array
-  body_awake: jax.Array
-  body_awake_ind: jax.Array
-  parent_awake_ind: jax.Array
-  dof_awake_ind: jax.Array
-  # position, velocity dependent:
-  flexedge_velocity: jax.Array
-  ten_velocity: jax.Array
-  actuator_velocity: jax.Array
-
-  qfrc_spring: jax.Array
-  qfrc_damper: jax.Array
-  subtree_linvel: jax.Array
-  subtree_angmom: jax.Array
-  qH: jax.Array  # pylint:disable=invalid-name
-  qHDiagInv: jax.Array  # pylint:disable=invalid-name
-  qDeriv: jax.Array  # pylint:disable=invalid-name
-  qLU: jax.Array  # pylint:disable=invalid-name
-  cacc: jax.Array
-  cfrc_int: jax.Array
-  cfrc_ext: jax.Array
-  # dynamically sized arrays which are made static for the frontend JAX API
-  # TODO(stunya): remove these dynamic fields entirely
-  contact: Contact
-  efc_type: jax.Array
-  efc_J: jax.Array  # pylint:disable=invalid-name
-  efc_pos: jax.Array
-  efc_margin: jax.Array
-  efc_frictionloss: jax.Array
-  efc_D: jax.Array  # pylint:disable=invalid-name
-  tree_island: jax.Array
-  map_itree2tree: jax.Array
-  efc_aref: jax.Array
-  efc_force: jax.Array
-
-
 class DataJAX(PyTreeNode):
   """JAX-specific data."""
 
@@ -1316,12 +1108,11 @@ class Data(PyTreeNode):
   qacc_smooth: jax.Array
   qfrc_constraint: jax.Array
   qfrc_inverse: jax.Array
-  _impl: Union[DataC, DataCPP, DataJAX, mjxw_types.DataWarp]
+  _impl: Union[DataCPP, DataJAX, mjxw_types.DataWarp]
 
   @property
   def impl(self) -> Impl:
     return {
-        DataC: Impl.C,
         DataCPP: Impl.CPP,
         DataJAX: Impl.JAX,
         mjxw_types.DataWarp: Impl.WARP,