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The histology and composition of muscles from normal (n = 10) and callipyge (n = 11) wether lambs was compared. Normal Rambouillet ewes were mated with callipyge Dorset rams, and their progeny were visually classified as callipyge or normal based on muscle definition in the loin and hind quarters. The muscles examined included three muscles that hypertrophy in callipyge lambs (semitendinosus, longissimus, and gluteus medius) and one muscle believed not to hypertrophy (supraspinatus). The hypertrophy-responsive muscles from callipyge lambs had a higher (P < .001) percentage of fast-twitch glycolytic (FG) fibers and lower (P < .001; P < .02 for SO in gluteus medius) percentages of slow-twitch oxidative (SO) and fast-twitch oxidative glycolytic (FOG) fibers. The diameters of the FG and FOG fibers were larger (P < .005 and P < .04, respectively) in hypertrophy-responsive muscles from callipyge lambs, but the SO fiber diameter was smaller (P < .05). Also, the protein:DNA ratio, an indicator of cell size, was greater (semitendinosus, P < .05); longissimus, P < .002; gluteus medius, P < .008) in the hypertrophy-responsive muscles from callipyge lambs. Thus, hypertrophy in callipyge lambs was, at least in part, due to fiber type changes and muscle cell enlargement. Hypertrophy was strongly associated with changes in the FG fibers, the only fiber type that increased in both proportion and average diameter in callipyge muscles. The protein:RNA ratio and RNA:DNA ratio, which are indicators of translational and transcriptional activity in the muscle cells, were not different between callipyge and normal muscles. This indicated that the accumulation of protein necessary for myofiber enlargement occurred without differences in the translational or transcriptional activity of callipyge muscle.