We compared the freeze-fracture morphology of the plasmalemma of the pectoralis major muscle taken from normal and dystrophic chickens at adult, embryonic, and early posthatching stages. Developmental changes were more conspicuous in surface caveolae than in intramembranous particles. The most striking differences between normal and dystrophic muscles were seen in the densities of the caveolae (17/micron2 in the normal adult; 30/micron2 in the dystrophic adult) and in their distribution (rectangular pattern in normals; random arrangement in dystrophics). These differences had already become significant at seven days posthatching and before the appearance of clinical symptoms. This is the earliest development stage at which morphologic abnormalities of plasmalemma have been reported for dystrophic muscle. Variations of surface topography and caveolar morphology, presumably representing secondary changes, were common in adult dystrophic muscle.