ALTHOUGH it was formerly assumed that the saturated and unsaturated fatty acids which comprise butterfat were straight-chained and possessed an even number of carbon atoms1, recent work has established that both branched-chain saturated acids and n-odd-numbered acids are also present. Of the saturated odd-numbered acids, n-undecanoic acid has been identified in hydrogenated butterfat, while n-tridecanoic and n-pentadecanoic acids have been shown to occur in butterfat which has not been subjected to hydrogenation. In this communication the isolation is reported from un-hydrogenated butterfat of n-heptadecanoic acid (margaric acid). Margaric acid has not previously been reported as a component of butterfat, although work in this laboratory has shown it to be present in mutton fat, in shark liver oil, and in hydrogenated ox perinephric fat. It has also been found in the free fatty acid fraction of human hair fat2. James and Martin3 recently reported that their gas-liquid chromatographic analyses indicated the probable presence of margaric acid in goat milk fat.