Lean meat and fat content of Australian sheep genotypes were compared at 20-, 30- and 40-kg carcass weights. Sheep comprised Poll Dorset_growth × Border Leicester Merino (PD_g × BLM), Poll Dorset_growth × Merino (PD_g × M), Poll Dorset_muscling × Merino (PD_m × M), Border Leicester × Merino (BL × M) and Merino × Merino (M × M) genotypes. Lambs were raised as a mixed flock under grazing and slaughtered at 4, 8, 14 and 22 months of age with each slaughter time involving ~150 mixed sex animals. At 24 h after slaughter, chilled carcasses were halved along the backbone and the right sides were used for determination of lean, fat and ash percentages using dual energy X-ray absorptiometry. Within a particular age group and genotype, animals growing at faster rates and reaching heavier carcass weights had lower carcass lean meat content than slower growing animals. Merino carcasses weighing 20 and 30 kg had similar levels of lean meat to PD × M genotypes, which was greater than that from the BL × M genotype. Second-cross PD × BLM carcasses weighing 20 kg at 4 months and 30 kg at 8 months had similar carcass fat and lean percentages to 20-kg Merino carcasses at 8 months and first-cross PD × M carcasses weighing 30 kg at 14 months, respectively. At 40-kg carcass weight, 22-month-old Merinos had similar levels of leanness to carcasses from 22-month-old PD × M animals and carcasses from 14-month-old second-cross PD × BLM animals. Carcass lean meat content decreased with increasing carcass weight and first- cross BL × M animals had the lowest carcass lean across all weight categories. There was a major acceleration in carcass fatness between 14 and 22 months associated with a reduction in muscle deposition. Results indicate that age of the animal should be taken into account when carcass lean and fat contents are compared at a particular carcass weight. Merinos will achieve weight/composition specifications at least equally well to crossbreds but will take longer with a likely increase in production costs.