diff --git a/requirements.txt b/requirements.txt
index 5c908d6..8d3ca4c 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -10,6 +10,6 @@ pytest-pythonpath>=0.7.2,<1
 setuptools>=38.4.0
 Sphinx>=2.2.0
 sphinx-rtd-theme>=0.4.3,<1
-tox>=3.0.0,<4
+tox==4.21.1
 twine==3.4.2
 pre-commit>=2.2.0