Hi Fernando, Matt Turk and I have been exchanging messages in the background and we are a bit puzzled about your problem, but have some ideas of where to ask you to look. Could you run this script, and give us the output (perhaps in a paste if it's long)? We're wondering if there are grids with particles in your disk, but without the star-specific fields. from yt.config import ytcfg; ytcfg["yt","serialize"] = "False" from yt.mods import * pf = load("DD0088/g1e10s1e11dm1e12_0088") my_disk=pf.h.disk([0.5, 0.5, 0.5], [0, 0, 1], 50/pf["kpc"], 10/pf["kpc"]) for grid in my_disk._grids: print grid.id, grid.NumberOfParticles try: ct = grid['creation_time'] print ct.size except: print 'no creation_time' Also, this command will uniquely pull out all the fields in your hdf5 files. It should be run in the same directory as the *cpu* files in DD0088. We're pretty sure that I was wrong about you missing particles, but this is a good double-check. It should include all the particle fields, including creation_time. h5ls -r *.cpu* | cut -f4 -d\/|cut -f1 -d\ |sort -u -- Stephen Skory s@skory.us http://stephenskory.com/ 510.621.3687 (google voice)