In various progressive muscular dystrophies (PMD), there were a number of fibers with histological and histochemical characteristics of regenerating fibers. With a morphometric analysis in diseased muscles, an identification of type 2C fiber on ATPase stain was the most simple and reliable method for fiber type evaluation. The type 2C fiber comprised 16.5% in Duchenne and 27.5% in Fukuyama type congenital MD, suggesting an active regenerating process taking place in both diseases. Regenerative capacity of muscle fibers after experimental damage to dystrophic chicken and mdx mouse muscles was similar to those seen in nondystrophic control muscles. From the above results, we speculate that a certain environmental factor such as interstitial fibrosis seems to play a major role in delaying regeneration in dystrophic muscles.