A sampling technique based on spaced serial ultrathin sections was used to examine the ultrastructure and cytoarchitecture of myofibers, which were necrotic throughout their length, in the dystrophic mutant mouse (C57Bl6J/dy2J). Regional variations along the length of the myofibers were described. The necrotic fibers extended the full length of the muscle fascicle and did not branch. In necrotic fibers which were free of invasive cells, the only nuclei found under the basal lamina were peripherally placed, euchromatic myonuclei, or nuclear membrane remnants. In these fibers, the nuclear population was approximately 14% of the nuclear population of "healthy" fibers from the same animals. No myosatellite cells were observed to be associated with these necrotic fibers. In necrotic fibers, which had undergone foreign cell invasion, the existence of a pleomorphic population of mononucleated invasive cells, many of which had not begun to express phagocytic characteristics, caused considerable confusion, because their appearance tended to mimic that of myonuclei or satellite cells. No heterochromatic nuclei or true myosatellite cells were found associated with necrotic fibers displaying macrophage invasion.