The degree of marbling and the thickness of subcutaneous fat at the 10th rib cut of the M. longissimus was studied in three groups of 450 kg Santa Gertrudis crossbred and Hereford steers fattened for 120 days on three grain-based diets in a commercial feedlot. Mean cold carcase weight was 311 kg for each breed of steer and differed by only 4 kg for each dietary treatment. The percentage intramuscular fat was similar for the two breeds of cattle but subcutaneous fat thickness was 25 per cent greater in the Herefords than in the Santa Gertrudis crossbreds. Visual marbling score was well correlated (r = 0.81) with the percentage intramuscular fat. Intramuscular adipocyte size was much less than subcutaneous adipocyte size and the Santa Gertrudis crossbreds tended to have larger subcutaneous adipocytes than the Herefords. These findings are discussed in relation to the differences between breeds of cattle in the rate of deposition and the distribution of carcase fat and the production of highly-marbled beef.