diff --git a/tox.ini b/tox.ini
index 7730d01e..71fae8d7 100644
--- a/tox.ini
+++ b/tox.ini
@@ -6,6 +6,7 @@ envlist = py27,py34,py35,pypy
 [testenv]
 deps = -rrequirements-dev.txt
 passenv = HOME HOMEPATH PROGRAMDATA TERM
+setenv = VIRTUALENV_NO_DOWNLOAD=1
 commands =
     coverage erase
     coverage run -m pytest {posargs:tests}