diff --git a/setup.py b/setup.py
index d03b64b..cdc2b7f 100755
--- a/setup.py
+++ b/setup.py
@@ -1,5 +1,6 @@
 #!/usr/bin/python3
 
+import os
 import sys
 
 from setuptools import setup, Extension
@@ -12,12 +13,14 @@
     package_dir={
         'cobs' : 'src/cobs',
     },
-    ext_modules=[
-        Extension('cobs.cobs._cobs_ext', [ 'src/ext/_cobs_ext.c', ]),
-        Extension('cobs.cobsr._cobsr_ext', [ 'src/ext/_cobsr_ext.c', ]),
-    ],
 )
 
+if not hasattr(sys, "pypy_version_info") and os.environ.get("COBS_PUREPYTHON"):
+    setup_dict['ext_modules'] = [
+                                    Extension('cobs.cobs._cobs_ext',    [ 'src/ext/_cobs_ext.c',  ]),
+                                    Extension('cobs.cobsr._cobsr_ext',  [ 'src/ext/_cobsr_ext.c', ])
+                                ]
+
 try:
     setup(**setup_dict)
 except KeyboardInterrupt: