diff --git a/tox.ini b/tox.ini
index ae86f10..d8526fd 100644
--- a/tox.ini
+++ b/tox.ini
@@ -1,5 +1,6 @@
 [tox]
 envlist = py26-django-14, py27-django-14, {py26,py27,py32,py33,py34,pypy}-django-{15,16,17,master}
+whitelist_externals = which
 
 [testenv]
 basepython =