Growth of beef bulls has been studied with the aid of the three genotypes, the Afrikaner, Hereford and Simmentaler. The study ranged from birth to about 80 weeks of age. Half of the bull calves were raised on a concentrate diet and the other half on a roughage diet. Measurements were continuously taken on live mass, voluntary intake and body composition using tritium dilution while intermittent measurements were made of apparent digestibility of feed energy (DE) at ad lib. intake. Genotype and age did not influence dietary DE%, the average on the concentrate diet was 68,4 ±2,2% and on the roughage diet 60,0 ± 5.3%. The growth results were analysed and interpreted relative to percentages of mature mass to account for differences in size. The intake of the Afrikaner on the concentrate diet was lower than that of the Hereford and Simmentaler and the same or even higher on the roughage diet. Growth on the other hand was less on both diets while that of the Hereford and Simmentaler were approximately the same. Consequently, efficiency in terms of both kg DM intake/kg gain in empty body mass and MJ ME intake/MJ gain in empty body energy was poorer for the Afrikaner than the other two breeds which again were very similar. As regards body composition, the Herefords were the fattest followed by the Afrikaner and Simmentaler, while the Simmentaler had the highest content of lean tissue and protein followed by the Afrikaner and the Hereford. The pattern of genotypic differences on the two diets was the same, but the calves were fatter on the concentrate diet, or the animals on the roughage diet had more lean tissue and protein. Since efficiency as defined here, was very simi1ar for the Hereford and Simmentaler irrespective of large differences in body composition, it was concluded that such differences account for only a relatively minor portion of the variation in efficiency. The major portion is accounted for by the energy loss from the system, i.e. energy which is not retained in the animal body, as demonstrated by the