Abstract Using Bar Harbor 129/ReJ dystrophic mice an in vitro study has been carried out in order to assess the capability of regenerating adult dystrophic muscle to proceed through myogenesis. An inability of dystrophic myoblasts to complete fusion was observed light-microscopically although the myoblasts became aggregated into muscle structures (called pseudostraps) bearing a superficial resemblance to normal multinucleate muscle straps. When investigated ultrastructurally the myoblasts constituting the pseudostraps were preserved in an immobile state by electron-dense attachment plaques. The location and structure of the dystrophic attachment plaques is described and compared with the plaques known to occur in normal myogenesis. Several additional myogenic features are described which occur consistently in pseudostrap myoblasts, including the formation of a basement membrane and elongation of the mitochondria. Well-developed myofibrillar protein synthesis could not be observed although swathes of filaments with primitive Z-bodies were present. A gross abnormal dilatation of the cisternae of the rough endoplasmic reticulum is described which is seen in approximately one-third of the dystrophic cultures. The results reported here are compared with those in which genetic variants of the Bar Harbor 129/ReJ dystrophic mouse have been used in coupled spinal cord and muscle cultures.