diff --git a/pyproject.toml b/pyproject.toml
index 3c98417e..49d542f2 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -1,4 +1,4 @@
 #pyproject.toml
 [build-system]
 # Minimum requirements for the build system to execute.
-requires = ['setuptools>=45.0', 'wheel', 'cython>=0.29', 'oldest-supported-numpy', 'mpi4py>=3.1.1']
+requires = ['setuptools>=45.0', 'wheel', 'cython>=0.29', 'oldest-supported-numpy', 'mpi4py==3.1.1']