diff --git a/build.py b/build.py
index 71c505256..01e20546b 100644
--- a/build.py
+++ b/build.py
@@ -13,10 +13,6 @@
 # See https://github.com/AspenWeb/pando.py/issues/542
 USE_PY_VENV = sys.version_info > (3, 3)
 
-ENV_ARGS = [
-    '-m', ('venv' if USE_PY_VENV else 'virtualenv')
-]
-
 
 def _virt(cmd, envdir='env'):
     envdir = _env(envdir)
@@ -52,8 +48,17 @@ def __env(envdir):
         # We've already built our own virtualenv.
         return envdir
 
-    args = [sys.executable] + ENV_ARGS + [envdir]
-    run(*args)
+    if USE_PY_VENV:
+        # use built-in venv module
+        run(sys.executable, '-m', 'venv', envdir)
+    else:
+        # use virtualenv instead
+        try:
+            import virtualenv
+        except ImportError:
+            # install it when missing
+            run(sys.executable, '-m', 'pip', 'install', '--user', 'virtualenv')
+        run(sys.executable, '-m', 'virtualenv', envdir)
     return envdir