--- a/python/mozbuild/mozbuild/virtualenv.py
+++ b/python/mozbuild/mozbuild/virtualenv.py
@@ -168,16 +168,21 @@ class VirtualenvManager(object):
         Receives the path to virtualenv's virtualenv.py script (which will be
         called out to), the path to create the virtualenv in, and a handle to
         write output to.
         """
         env = dict(os.environ)
         env.pop('PYTHONDONTWRITEBYTECODE', None)
 
         args = [python, self.virtualenv_script_path,
+            # Without this, virtualenv.py may attempt to contact the outside
+            # world and search for or download a newer version of pip,
+            # setuptools, or wheel. This is bad for security, reproducibility,
+            # and speed.
+            '--no-download',
             self.virtualenv_root]
 
         result = subprocess.call(args, stdout=self.log_handle,
             stderr=subprocess.STDOUT, env=env)
 
         if result:
             raise Exception(
                 'Failed to create virtualenv: %s' % self.virtualenv_root)