Histological serial sections of fetal stages of various rodent taxa have been studied. We have concentrated on the ontogenetic differentiation of the infraorbital region in some hystricomorphs and myomorphs. The glirid taxa Graphiurus, Eliomys, Glis and Muscardinus show a specific mode of development of the medial masseter muscle that is clearly different from the other groups: In early ontogenetic stages, the muscle invariably has its anterior-most origin at the dorsolateral rim of the infraorbital foramen, and only in later fetal stages migrates to the side of the muzzle. In contrast to the glirines, Graphiurus has an aboral origin of the lateral masseter muscle from the beginning, and we consider this as a plesiomorphic state. In all other taxa showing an enlarged masseter medialis, this muscle bundle originates from the anterior portion of the ascending process or even from the premaxillary. We interpret our ontogenetical findings as support for the hypothesis of the monophyly of the Gliroidea and for the concept of `pseudo-myomorphy' of Vianey-Liaud (1985), which was derived from the fossil record. This evidence suggests that the gliroids are probably not members of the Myomorpha, but have acquired myomorphy independently. This conclusion is also in agreement with many molecular studies that suggest closer affinities of glirids with sciurids.