The internal architecture of the masticatory muscles in a murine murid, Rattus rattus, was examined and compared with our previous description of Peromyscus maniculatus, which seems to have primitive jaw muscles for muroid rodents. Remarkable differences were observed mainly in the masseter muscle and corresponding aponeurosis. The anterior and posterior mandibular aponeuroses for insertion of the masseter deep layer are united as a single sheet in Rattus. In Peromyscus, both the origin and insertion of the anterior part of the masseter deep layer lie on the aponeurosis. In Rattus, however, the deep fibers of the anterior part form a muscular attachment to the bone surface at either end. This characteristic suggests that the maximum angle of gape is increased in murine murids, which allows them to ingest various-sized foods.