diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 757eaa44510..98c2ec0a22e 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -161,6 +161,8 @@ To build all libraries and tests, with Python packages in development mode, simp
 ./build.sh --pydevelop libcudf libcudf_kafka cudf dask_cudf cudf_kafka custreamz
 ```
 
+- **Note**: if Cython files (`*.pyx` or `*.pxd`) have changed, the Python build must be rerun.
+
 To run the C++ tests, run
 
 ```bash