Compared with pigs, cattle and sheep lay down much of their carcass fat between the muscles (intermuscular fat) and a smaller proportion on the carcass outer surface (subcutaneous fat). The skin is also removed from cattle and sheep during the dressing process, and this results in surface irregularities on the subcutaneous fat. Because of these species differences, simple methods of estimating carcass lean proportion, based on the depth of subcutaneous fat, have been less successful in ruminants. In particular, there has been no counterpart in the beef and sheep carcass classification schemes to the optical probe used so successfully in the pig sector. But there is a need for an objective method of classifying beef and sheep carcasses according to their lean meat proportion, and in this study the speed of ultrasound technique was examined as a possible contender for this role.