The sartorius muscles of normal and dystrophic chicken at different ages were compared by histochemical and electron microscopic techniques. In the normal muscle type I and type II fibers with three subtypes of the latter were present at the adult stage. At hatching the two main fiber types could be differentiated; the subtypes, however, did not differentiate until about three weeks. In the sartorius of adult dystrophic chicken, though some fibers appeared atrophic, the striking change was hypertrophy of type II fibers. The reactions of fiber types were normal except for some increase of succinic dehydrogenase staining. The pattern of fiber type differentiation was also unaltered; the earliest change in the dystrophic sartorius appeared to be increased production of muscle fibers.