diff --git a/tox.ini b/tox.ini
index bc037775..372dc5c5 100644
--- a/tox.ini
+++ b/tox.ini
@@ -63,6 +63,7 @@ deps = unittest2
 
 [testenv:py26-full]
 basepython = python2.6
+setenv = TORNADO_EXTENSION=1
 deps =
      {[testenv:py26]deps}
      futures
@@ -72,6 +73,7 @@ deps =
 
 [testenv:py27-full]
 basepython = python2.7
+setenv = TORNADO_EXTENSION=1
 deps =
      futures
      mock
@@ -90,12 +92,14 @@ commands = python -bb -m tornado.test.runtests {posargs:}
 
 [testenv:py32-full]
 basepython = python3.2
+setenv = TORNADO_EXTENSION=1
 deps =
      mock
      pycurl>=7.19.3
 
 [testenv:py33-full]
 basepython = python3.3
+setenv = TORNADO_EXTENSION=1
 deps =
      pycurl>=7.19.3