The effect of dietary levels of phosphorus on the lipid content and its composition in carp was investigated. The diets containing different levels (3.2 to 1, 880mg per 100g) of phosphorus were fed for 6 weeks, and then some of the carp were kept without feeding for 3 weeks. On the termination of feeding and non-feeding periods, moistrue and crude fat were determined. The lipids were fractionated into polar and nonpolar lipid, and the fatty acid composition of both lipids were determined. The dietary levels of phosphorus affected the moisture and crude fat in carcass and viscera. Feeding low-phosphorus diet resulted in low moisture and high crude fat. Carp fed on the diet containing half the amount of phosphorus requirement contained 2 times as much fat as those fed on the diet of sufficient amount of phosphorus. In the carp fed on low-phosphorus diet, the amount of oleic acid of nonpolar lipid was more than that of carp fed on the diet containing sufficient amount of phosphorus. During the non-feeding period of 3 weeks, the crude fat of entire fish ingested sufficient phosphorus decresed to two or one third but that of deficient group did not decrease. The decrease of body weight by non-feeding in the deficient group was larger than that of the carp fed on diet containing sufficient amount of phosphorus before non-feeding. It as contrary to the decrease of crude fat. The results obtained suggested that β-oxidation of fatty acid is inhibited by deficiency of dietary phosphorus.