Total lipids, triglycerides, and phospholipids from several lots of beach-spawning capelin and one lot of prespawning, offshore capelin have been determined and examined in detail for fatty acid composition. Generally, but not invariably, female fish contain more fat, the difference being apparent in the triglycerides. These are also usually of higher iodine value than triglycerides in the males. During frozen storage the lipids, including triglycerides, in most samples of male capelin undergo more severe hydrolysis than in the females. It is believed that these phenomena are related to bioenergetic relationships by which the males require very rapid catabolism of depot fat and hence nonspecific hydrolysis of fatty acids, whereas the less active females show some selectivity and preferentially catabolize the longer-chain monounsaturated fatty acids.When allowances are made for 16:1 and 18:1 being interchangeable, Newfoundland capelin triglycerides are shown to be essentially similar to eastern Atlantic commercial capelin oil, and may be distinguished by certain composition characteristics from other Canadian commercial oils of comparable iodine value. Details of fatty acid compositions are summarized for nutritional evaluation of capelin and capelin products.