Two genetic strains of white turkeys were reared together in a large flock under conditions allowing optimal growth from hatching to 20 weeks. Birds of both strains followed similar growth curves with males of both strains growing heavier than females. In one strain, however, the yield of breast meat as a proportion of live weight was greater in both males (287 v. 225 g/kg) and females (276 v. 201 g/kg). When expressed in ratio to breast width, maximum meat depth was greater in the strain which yielded the higher proportion of breast meat. However, with this index of breast plumpness, females scored higher than males in both strains (high yield, 0·45:1·00 v. 0·37:1·00; low yield, 0·33:1·00 v. 0·31:1·00). In the superficial anterior region of the pectoralis muscle in both strains and sexes, there were strong adenosine triphosphatase and weak succinate dehydrogenase reactions in nearly all muscle fibres. Differences in the radial growth of muscle fibres were found between the two strains and, by 20 weeks, birds of the lower yield strain had reached only 0·85 (males) or 0·82 (females) of the mean diameters attained in birds of the higher yielding strain.