We here describe a near-complete dentition, partial rostrum, and associated lumbar vertebrae of the multituberculate mammal Cimexomys judithae, discovered on a dinosaur nest horizon at Egg Mountain, in Montana's Campanian Two Medicine Formation. This specimen confirms associations of isolated teeth made by previous authors and provides a wealth of phylogenetically useful information. A phylogenetic analysis including this specimen indicates that Cimexomys is diagnosed on the basis of primitive and widely distributed character states, is probably not monophyletic, and that C. judithae is more derived than other species of Cimexomys. Species of Cimexomys are too primitive to be included in the suborders Taeniolabidoidea or Ptilodontoidea, and are thus useful as outgroups in future analyses, but they must be scored separately to insure monophyly.