Geoffrey, I think we've stumbled upon a h5py bug! Without getting into gritty details, the LoadHaloes() code actually opens, closes, and then re-opens the h5 file when data is read in. For some reason, the re-open wasn't working, and the file was still marked as closed, and therefore trying read data from it wasn't working. I've added a band-aid of sorts which seems to fix the issue. Go ahead and update and let me know if you continue to have problems. -- Stephen Skory s@skory.us http://stephenskory.com/ 510.621.3687 (google voice)