Orthotopic transplants of whole extensor digitorum longus muscles were performed on six 4-6-week-old 129 ReJ mice. One hundred days posttransplantation, the animals were killed and the regenerated muscles were processed for electron microscopy. The grafts contained polygonal-shaped myofibers with persistent central nuclei, organized into discrete muscle fascicles. No central area of fatty infiltration or fibrosis was observed. The mean number of myofibers in a regenerating transplanted muscle, as determined from an ultrathin section taken from the graft's widest girth, was 631 (SEM = +/- 59), a reduction of approximately 32% from that found in age-matched control muscle (Ontell et al., 1983). By following the myofibers in spaced, serial ultrathin sections along their length, it was found that the branched, regenerating myofibers found in immature grafts of normal muscle (Ontell et al., 1982) persisted in stabilized, long-term transplanted muscle. The frequency of branching was determined by following each fiber found at the widest girths of four of the grafts in spaced, serial ultrathin sections (15-micron intervals) for approximately 2% of the total length of the grafts. Over this distance, 6.6% of the fibers were involved in the branching phenomenon. The persistence of branched fibers in long-term grafts and the frequency with which the branching phenomenon was found to occur may have physiological consequences and should be investigated.