diff --git a/tox.ini b/tox.ini
index b98feaea..db0106c9 100644
--- a/tox.ini
+++ b/tox.ini
@@ -1,5 +1,6 @@
 [tox]
 envlist = py35, py36, pep8, docs
+minversion = 2.3.1
 # Allows docs to be built without setup.py having to exist. Requires that
 # usedevelop be False as well (which it is by default).
 skipsdist = True
@@ -11,23 +12,27 @@ setenv = VIRTUAL_ENV={envdir}
 passenv = http_proxy HTTP_PROXY https_proxy HTTPS_PROXY no_proxy NO_PROXY
 
 [testenv:py35]
+basepython = python3.5
 commands =
     # Run all unit tests under src/bin/pegleg
     tox -c src/bin/pegleg/tox.ini -e py35
 whitelist_externals = tox
 
 [testenv:py36]
+basepython = python3.6
 commands =
     # Run all unit tests under src/bin/pegleg
     tox -c src/bin/pegleg/tox.ini -e py36
 whitelist_externals = tox
 
 [testenv:fmt]
+basepython = python3
 commands =
     tox -c src/bin/pegleg/tox.ini -e fmt
 whitelist_externals = tox
 
 [testenv:pep8]
+basepython = python3
 commands =
     tox -c src/bin/pegleg/tox.ini -e pep8
 whitelist_externals = tox