diff --git a/build/virtualenv/populate_virtualenv.py b/build/virtualenv/populate_virtualenv.py
index adbc9c733766..e734021521d0 100755
--- a/build/virtualenv/populate_virtualenv.py
+++ b/build/virtualenv/populate_virtualenv.py
@@ -193,7 +193,8 @@ class VirtualenvManager(object):
         # configure or a mozconfig activated in the current shell. We trust
         # Python is smart enough to find a proper compiler and to use the
         # proper compiler flags. If it isn't your Python is likely broken.
-        IGNORE_ENV_VARIABLES = ('CC', 'CXX', 'CFLAGS', 'CXXFLAGS', 'LDFLAGS')
+        IGNORE_ENV_VARIABLES = ('CC', 'CXX', 'CFLAGS', 'CXXFLAGS', 'LDFLAGS',
+            'PYTHONDONTWRITEBYTECODE')
 
         try:
             old_target = os.environ.get('MACOSX_DEPLOYMENT_TARGET', None)