diff --git a/chex/_src/asserts_chexify.py b/chex/_src/asserts_chexify.py
index ff9565f5..1882b25d 100644
--- a/chex/_src/asserts_chexify.py
+++ b/chex/_src/asserts_chexify.py
@@ -20,6 +20,7 @@
 import dataclasses
 import functools
 import re
+import threading
 from typing import Any, Callable, FrozenSet
 
 from absl import logging
@@ -178,6 +179,8 @@ def logp1_abs_safe(x: chex.Array) -> chex.Array:
     thread_pool = futures.ThreadPoolExecutor(1, f'async_chex_{func_name}')
     # A deque for futures.
     async_check_futures = collections.deque()
+    # Protect the futures from concurrent access.
+    async_check_futures_lock = threading.Lock()
 
   # Checkification.
   checkified_fn = checkify.checkify(fn, errors=errors)
@@ -191,8 +194,9 @@ def _chexified_fn(*args, **kwargs):
 
     if async_check:
       # Check completed calls.
-      while async_check_futures and async_check_futures[0].done():
-        _check_error(async_check_futures.popleft().result(async_timeout))
+      with async_check_futures_lock:
+        while async_check_futures and async_check_futures[0].done():
+          _check_error(async_check_futures.popleft().result(async_timeout))
 
     # Run the checkified function.
     _ai.CHEXIFY_STORAGE.level += 1
@@ -214,8 +218,9 @@ def _chexified_fn(*args, **kwargs):
 
   def _wait_checks():
     if async_check:
-      while async_check_futures:
-        _check_error(async_check_futures.popleft().result(async_timeout))
+      with async_check_futures_lock:
+        while async_check_futures:
+          _check_error(async_check_futures.popleft().result(async_timeout))
 
   # Add a barrier callback to the global storage.
   _ai.CHEXIFY_STORAGE.wait_fns.append(_wait_checks)
diff --git a/chex/_src/asserts_chexify_fuzz_test.py b/chex/_src/asserts_chexify_fuzz_test.py
new file mode 100644
index 00000000..8e9f3a48
--- /dev/null
+++ b/chex/_src/asserts_chexify_fuzz_test.py
@@ -0,0 +1,66 @@
+# Copyright 2020 DeepMind Technologies Limited. All Rights Reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+# ==============================================================================
+"""Fuzz test for `asserts_chexify.py`."""
+
+import concurrent.futures
+import random
+import time
+
+from absl.testing import absltest
+from chex._src import asserts
+from chex._src import asserts_chexify
+from chex._src import variants
+import jax
+import jax.numpy as jnp
+
+
+class AssertsChexifyFuzzTest(variants.TestCase):
+  """Fuzz test for thread safety of chexify."""
+
+  def test_thread_safety(self):
+
+    def assert_negative():
+      result = jnp.ones(shape=())
+      # This assert will always fail.
+      asserts.assert_scalar_negative(result)
+      return result
+
+    def chexified_assert_negative():
+      fn = asserts_chexify.chexify(assert_negative, async_check=True)
+      fn()
+      # Introduce random delay between the two calls, otherwise we will not
+      # get interleaving of the two operations between threads because they
+      # happen too quickly.
+      time.sleep(random.uniform(0.01, 0.02))
+      asserts_chexify.block_until_chexify_assertions_complete()
+
+    with concurrent.futures.ThreadPoolExecutor(max_workers=16) as executor:
+      futures = []
+      for _ in range(1000):
+        future = executor.submit(chexified_assert_negative)
+        futures.append(future)
+
+      for future in concurrent.futures.as_completed(futures):
+        try:
+          future.result()
+        except AssertionError:
+          pass
+
+    asserts_chexify.block_until_chexify_assertions_complete()
+
+
+if __name__ == '__main__':
+  jax.config.update('jax_numpy_rank_promotion', 'raise')
+  absltest.main()