diff --git a/tox.ini b/tox.ini
index 2bd7edfe5..3d1a18c69 100644
--- a/tox.ini
+++ b/tox.ini
@@ -10,6 +10,9 @@ commands =
     python setup.py develop easy_install letsencrypt[testing]
     python setup.py test -q # -q does not suppress errors
 
+setenv =
+    PYTHONPATH = {toxinidir}
+
 [testenv:cover]
 basepython = python2.7
 commands =