The growth and distribution of intramuscular fat was studied in 62 carcases from four beef genotypes, Angus, Hereford, Friesian and Charolais x Illawarra. Relative to dissected side fat weight, ether extract weight of side muscle was deposited in a similar manner in all four genotypes. At high levels of side fatness (45 kg) Herefords had significantly less ether extract than the other genotypes. Ether extract percentage (of side muscle) was greater in the Angus than in the other genotypes at low side fatness levels (20 kg, 30 kg or 20%). Relative to dissected fat percentage ether extract percentage of side muscle, initially lower in the Friesians and Charolais x Illawarra than in the Angus and Herefords at low levels of carcase fatness, was not significantly different from them over 27% dissected fat. With increasing carcase fatness marbling fat was deposited differentially among the primal cuts, with few major between-breed differences. A study of the muscle weight distribution of the primal cuts confirmed the cutting technique as a useful method for examining the commercial importance of marbling.