The blubber, liver, and muscle of the Antarctic sei whale were analyzed for total lipid content, composition of lipid by classes and positional distribution of fatty acids in individual lipids. The major glycerolipids (triglycerides, phosphatidylcholine, and phosphatidylethanolamine) were fractionated by silver nitrate thin layer chromatography. The phospholipid fractions were analyzed for fatty acid positional distribution. The whale stomach contained almost exclusively the amphipod Parathemisto gaudichaudi. Its lipids were also studied and compared with the lipids of the body tissues. The results indicate that the stomach content lipids are subjected to modifications before being deposited in the blubber, liver, and muscle. According to the silver nitrate thin layer chromatographic studies, liver and blubber triglycerides resembel each other in their patterns of positional distribution of fatty acids and in molecular species composition. The phospholipids of liver and blubber also exhibited closely related fatty acid distribution patterns. In general, while the proportions of lipid classes and their predominant fatty acids varied from tissue to tissue, the patterns according to which the lipids had been synthesized seemed to be common.