diff --git a/tox.ini b/tox.ini
index d834512..fcdbdde 100644
--- a/tox.ini
+++ b/tox.ini
@@ -3,4 +3,4 @@ envlist = py27,py34
 
 [testenv]
 deps = -rtests/test_requirements.txt
-commands = py.test tests/
+commands = py.test {posargs:tests/}