Although several studies have been made of the ultrastructural changes of muscular dystrophy in both the human and the mouse, there has been little or no attempt to correlate these changes with the conventional histopathological features nor has there been a complete description of the fine structural alterations. A number of different theories concerning the basic ultrastructural changes have been proposed. According to van Breeman (1, 2), vacuolation of the sarcoplasmic reticulum is the fundamental abnormality. Mölbert (3, 4), on the other hand, considered the myofibrils to be affected primarily. Pearce and Walton (5, 6) and Pearce (7, 8) have described varying degrees of abnormality of all elements of the muscle fiber, particularly of the lysosomes and the sarcoplasmic reticulum. Ross et al(9, 10) concluded, from their studies of dystrophic mice, that the principal alterations occurred in the myofibrils, the sarcoplasmic reticulum, and the mitochondria. Still others (11, 12) have postulated that a defective sarcolemma constituted the initial defect. Many of the ultra-structural changes which characterize muscular dystrophy have been clearly recognized in these studies, but there has been no mention of several other changes which appear to be of fundamental importance. The conspicuous abnormalities in our material have been the spontaneous discontinuous, embryonal form of regeneration of muscle fibers as well as an unusual type of degeneration which results in a longitudinal division of the myofibrils. Both of these changes as well as other fine structural features of the two-week-old dystrophic mouse will be emphasized in this report.