An inherited muscle hypertrophy in sheep is caused by the callipyge gene (CLPG) located on ovine chromosome 18. It has been suggested that this gene is a single autosomal dominant gene. Animals expressing the callipyge gene show increased muscling in the pelvic and torso regions of the body and, thus, have been given the phenotype- descriptive name callipyge (from Greek calli-beautiful; pyge-buttocks). In this study 21 wether lambs, the offspring of callipyge rams (genotype CLPG /clpg) and normal Rambouillet ewes (genotype clpg/clpg), were used to determine the difference s in muscle fiber type percentages, composition, and tenderness between normal and callipyge lambs . Eleven of these lambs showed muscle hypertrophy typical of the callipyge phenotype; 10 were classified as normal . Several histochemical, biochemical , and physical measures were examined in order to study changes in the physiology and biochemistry of some economically important muscles. When compared to normal lambs, the callipyge lambs have a larger (P < .05) average percentage of fast-twitch glycolytic (FG) muscle fibers and smaller average percentages of fast-twitch oxidative and glycolytic (FOG) and slow-twitch oxidative (SO) muscle fibers in both the longissimus and gluteus medius muscles. The diameter of the fast-twitch muscle fibers was larger in the callipyge group, but slow-twitch fibers were smaller than those of normal lambs. No differences were observed in the supraspinatus muscle of the normal and callipyge groups. Thus there is an indication of a differential effect of the callipyge gene among muscles of the callipyge lambs. The semitendinosis muscles of both the callipyge and normal groups were dissected from the carcasses. This muscle was larger (P < .01) and contained more protein in the callipyge lambs than in the normal lambs. However, the callipyge semitendinosis muscle did not have a significantly higher content of DNA than the normal lamb semitendinosis, suggesting that the muscle hypertrophy is not associated with an increase in muscle nuclei. The protein-to-DNA ratio was larger (P < .05) in the semitendinosis muscle of callipyge lambs than in the normal lambs. Protein-to-RNA and RNA-to-DNA ratios were similar; this suggests that the semitendinosis muscle was enlarged without increased translational or transcriptional activity. Samples from the callipyge longissimus and gluteus medius muscles had RNA, DNA, and protein ratios similar to those of the semitendinosis muscle, suggesting a similar mode of action for muscle enlargement in other muscles affected by this gene. Loin chops from the callipyge lambs had lower tenderness scores (P < .01) as measured by the Wamer-Bratzler shear force and myofibril fragmentation index (MFI). However, aging increased MFI scores and decreased shear scores (P < .0 l) of the callipyge lamb chops . The normal lamb chops also had decreased shear and increased MFI scores following the aging period. The loin chops from the callipyge lambs also tended to be less red (P < .1) than chops from normal animals as measured by Hunter 'L,' 'a,' and 'b' colorimeter scores.