The ultrastructure and the three-dimensional cytoarchitecture of the developing murine extensor digitorum longus muscle has been studied in spaced, serial, transverse and longitudinal ultrathin sections of the muscles of 12-, 14-, 16-, and 18-day in utero, newborn, and 5-day-old 129 ReJ mice. Despite the fact that in vivo myogenesis is asynchronous (i.e., during most of the fetal period, multiple stages of myogenesis can be seen in a single developing muscle mass), a distinct temporal pattern of development can be seen across the entire width and length of the developing muscle. At 12 days in utero, the developing extensor digitorum longus muscle consists of primary myotubes surrounded by a pleomorphic population of mononucleated cells devoid of myofilaments. At this stage, blood vessels and nerves are found peripheral to but not within the developing muscle mass. A delay of 2 days occurs between the time of formation of the primary and secondary myotubes. Clusters (consisting of one primary myotube and secondary myotubes), axon bundles, capillaries, and primitive motor endplates are found in the muscle by 16 days in utero. Evidence is presented consistent with the hypothesis that cluster formation and cluster dispersal occur simultaneously in the developing muscle, beginning as early as 16-days in utero. By 18 days in utero, many of the primary myotubes of the cluster and the independent myotubes (i.e., single myotubes enclosed in their own basal lamina) have begun to acquire the polygonal shape, fascicular arrangement, and ultrastructure characteristic of more mature myofibers. At birth, clusters are infrequently encountered, and intramuscular axons have begun to undergo myelination. At this time, the only undifferentiated, mononucleated cells present in the muscle are myosatellite cells. The first week postnatal was characterized by further maturation of the myofibers.