diff --git a/tox.ini b/tox.ini
index 36983ee5..4c454ed2 100644
--- a/tox.ini
+++ b/tox.ini
@@ -6,6 +6,7 @@ deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 setenv=
   PYTHONWARNING=all
+basepython=python3.5
 commands=
   pytest \
     {posargs}