Myosin, the protein responsible for ATPase activity and hence the conversion of chemical into mechanical energy by muscle, is known to exist in polymorphic forms. A correlation exists between the myosin type present in a muscle as demonstrated by gel electrophoresis and the staining of sections of that muscle for ATPase activity. It is still possible to determine the major fibre type in a muscle electrophoretically even when pathological changes make interpretation of histochemical staining of sections difficult. The refinement of the electrophoretic technique in the present study has separated myosin from some specimens of muscle into three isoenzyme forms. The density of staining of the three myosin bands corresponds to the numbers of muscle fibres staining dark, pale or intermediate grey for ATPase activity after preincubation at pH 4.6. As other workers have noted, some specimens of muscle show a continuum or wide range of intermediate staining fibres after preincubation at pH 4.6. However, this is not reflected in the electrophoretic patterns which show only the three isoenzyme forms of myosin rather than a large range of these molecules. A particular myosin molecule would appear to be specific for each of the histochemical fibre types 1, 2A and 2B.