Okay, folks, I confirmed that this was indeed because a package named "elisa" was installed on the user's system, and I added this work- around to my package's setup.py: http://allmydata.org/trac/pycryptopp/browser/setup.py?rev=567#L146 146 try: 147 _setup(test_suite="pycryptopp.test") 148 except BaseException, le: 149 # to work around a bug in Elisa 150 if "test_suite must be a list" in str(le): 151 _setup(test_suite=["pycryptopp.test"]) This works on both my system (with setuptools 0.7 dev) and my user's system (with setuptools 0.6c8 and elisa). I will try to contact the authors of elisa. Regards, Zooko --- http://allmydata.org -- Tahoe, the Least-Authority Filesystem http://allmydata.com -- back up all your files for $5/month