diff --git a/tox.ini b/tox.ini
index 16d4d16..a623152 100644
--- a/tox.ini
+++ b/tox.ini
@@ -1,6 +1,10 @@
 [tox]
 minversion = 2.0
 envlist = py27,py37,pep8,docs
+# Pin setuptools via virtualenv as latest virtualenv pulls in 65.3.0
+# version of setuptools for py37, which causes installation problems
+# (virtualenv 20.16.3 uses setuptools 63.4.1).
+requires = virtualenv<20.16.4
 
 [testenv]
 install_command = pip install {opts} {packages}