Growth of four muscles was studied in four lines of homozygous dystrophic chickens selected for high or low fat content of pectoralis muscles, or for early or late onset of disability. The light (“white”) pectoralis, supracoracoideus and biceps brachii and the dark (“red”) adductor muscles were studied. Growth characteristics were compared to a randomly mated dystrophic line and to normal chickens. Significant differences in muscle size occurred between lines but all dystrophic lines underwent the same sequence of events: gross hypertrophy of the white muscles during the first few weeks of life, followed enventually by gross atrophy. The dark adductor muscles did not differ consistently from normal. Hypertrophy was accompanied by increased protein (or fat-free dry matter) and water content. Atrophy was accompanied by increased fat content. Selected lines exhibited greater uniformity and greater extremes of hypertrophy or atrophy than occurred in the random line.