Bulls and steers from Afrikaner and Friesland cattle fed ad lib. were slaughtered at different ages from birth to 24 months inclusive. Samples were taken from the M. semimembranosus and M. semitendinosus (dark and light) and histochemically treated with succinic dehydrogenase f’or red and white muscle fibre identification. Muscle fibre diameter was also determined. No grouping into any particular pattern of the muscle fibre types could be observed. A gradual transition from red to white muscle fibre types occurs with age. The Friesland has a larger percentage white muscle fibres and a lower percentage red muscle fibres than the Afrikaner for the muscles concerned. Bulls have a larger percentage red muscle fibres and a lower percentage white muscle fibres than the steers The increase in muscle fibre diameter follows a characteristic pattern for the particular muscles. The M. semimembranosus and the M. semitendinosus (dark area) show an increase in diameter up to l2 months before tapering off while the M. semitendinosus (light area) shows an increase up to 20 months before levelling off. White muscle fibre diameter is larger than red muscle fibre diameter at all ages The Afrikaner bulls have a smaller mean fibre diameter than Friesland bulls. The bulls, however, have a larger mean muscle fibre diameter than the steers notwithstanding the lower percentage of white muscle fibres. The possible relationship between the type of muscle metabolism and meat quality characteristics is discussed.