Hi, For ART particle data yt returns incorrect mass values. I believe this is something Matt had solved in March by dividing mass units by ng**3 for particle mass data. Miguel Rocha brought this to my attention and I have been able to reproduce the problem on the public ART dataset (D9p_500). http://paste.yt-project.org/show/4908/ I thought we had included the following in art/frontends/fields.py, but am unable to find it now. def setup_particle_fields(self, ptype): self.add_field((ptype, "particle_mass"), function=_particle_mass, units="code_mass", particle_type=True) super(ARTFieldInfo, self).setup_particle_fields(ptype) Why would this have been working at some point and now it is broken again? All the best, Kenza