Using capillary GLC we analyzed the plasma fatty acids in a group of full-blood Aborigines in north western Australia before and after 2 wk on a diet in which over 90% of the energy was derived from tropical fish and shellfish. The proportion of saturated fatty acids did not change and all monoenoic and omega 6 fatty acids, except arachidonic, fell significantly. The proportions of arachidonic and all omega 3 PUFA rose significantly on the diet. This striking rise in arachidonic was evident in all lipid fractions (phospholipids, cholesterol esters, and triglycerides). Total triglycerides in fasting plasma fell from 1.32 to 0.61 mM after the diet while total cholesterol, which was low initially, did not fall significantly. Analysis of the fatty acids in lipid extracts from the tropical seafood eaten in the study revealed an arachidonic acid content ranging from 4.8 to 14.3% of the total fatty acids. The seafood contained almost no linoleic acid but was, as expected, a rich source of omega 3 fatty acids (13.6 to 31.0% of the total fatty acids). From these data we are able to conclude that seafood from tropical waters, unlike seafood from colder waters, is a natural source of polyunsaturated fatty acids from both the omega 6 and omega 3 series.