diff --git a/tox.ini b/tox.ini
index d9d1e5b..12f26a8 100644
--- a/tox.ini
+++ b/tox.ini
@@ -1,5 +1,5 @@
 [tox]
-envlist = py27, py34, py35
+envlist = py27, py33, py34, py35
 
 [testenv]
 commands =