According to Michael Devitt, the primary work of a rigidity distinction for kind terms is to distinguish non-descriptional predicates from descriptional predicates. The standard conception of rigidity fails to do this work when it is extended to kind terms. Against the standard conception, Devitt defends rigid application: a predicate is a rigid applier iff, if it applies to an object in one world, it applies to that object in every world in which it exists. Devitt maintains that rigid application does the job of identifying nondescriptional predicates perfectly. I argue that Devitt is wrong about this. When we examine more closely alternative theories about the identity and persistence conditions of those entities to which mass terms apply, we find no plausible theory that has the result that a term is rigid iff it is non-descriptional. © 2012 Springer Science+Business Media Dordrecht.