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Muscle differentiation was studied in foetal Semitendinosus muscle from normal cattle and those with the 'culard' gene of muscular hypertrophy sampled at 90, 130, 170 and 210 days of foetal life. The different fibre types were characterized by immunohistochemistry with antibodies specific to different isoforms of myosin heavy chains. The isoforms were separated by electrophoresis, identified by immunoblotting and quantified by ELISA. In double-muscled animals, there was a slower rate of differentiation in the first generation of cells, most markedly apparent at 90 days. At 130 days, differentiation was retarded mainly in the second generation, while at 170 days there was no longer any difference between the two animal types in the differentiation of first generation cells, which were totally slow in both. At the same stage however, type IIC fibres in double-muscled animals were much slower in appearing and continued to be so at 210 days, albeit to a lesser extent. These findings show that differentiation of the muscle fibres occurs at a slower rate in double-muscled foetuses particularly during the first two-thirds of foetal life.