Sorry for asking again. I know that Accelerate lapack has been deprecated. I have openblas from homebrew. If I use LDFLAGS, CPPFLAGS, and PKG_CONFIG_PATH , I get a very long traceback of compiler errors, I can post them if that helps. Without the flags, I get a segfault at test_cython_blas. I am prepending the flags inline to runtest.py instead of exporting them, if that makes a difference.