The biochemical assay of certain metabolites of the red and white myotomal muscles of the tuna,Euthynnus affims Cantor has been carried out. The metabolites exhibited a marked variation in their distribution pattern in red and white muscles. The narrow red fibres are characterised by higher levels of lipid, glycogen, myoglobin and SDH while the broader white fibres had lesser amount of the above metabolites. The distribution of metabolites—the myoglobin and SDH, revealed a gradientfrom the superficial towards the inner layers of the red myotomal muscle in both the pectoral and middle regions. The physiological relevance of these biochemical variations in diverse regions of the red and white muscle is discussed.