The histochemical and biochemical myofibrillar ATPase activities of red and white muscle of the catfish and mullet have been investigated. The histochemical reactions confirmed the typical pattern of red and white muscle and the absence of growing fibers in the white muscle of the mullet. Biochemical assay were used to determine the ATPase activity of red and white muscle myofibrils. Ratios of white muscle activity/red muscle activity were found to 2.2 in the mullet and 2.3 in the catfish. The myofibrillar ATPase activity of mullet was less thermostable than that of catfish but is still in the range for fish leaving in warm waters.