diff --git a/tox.ini b/tox.ini
index 2f2b1757..815d2acc 100644
--- a/tox.ini
+++ b/tox.ini
@@ -7,6 +7,6 @@ commands =
 deps =
     nose
     mg35: PyMongo==3.5
-    mg3x: PyMongo>=3.0
+    mg3x: PyMongo>=3.0,<3.7
 setenv =
     PYTHON_EGG_CACHE = {envdir}/python-eggs