Geoffrey, > name 'fKD' is not defined did you upgrade your yt? You need to build the fKDpy.so file: http://yt.enzotools.org/doc/extensions/running_halofinder.html#fortran-kd-tree-setup Does that fix it? Stephen Skory s at skory.us http://stephenskory.com/ 510.621.3687 (google voice)