The lipid composition of two species of Serrasalmid fish with different natural feeding habits were compared in relation to the polyunsaturated fatty acids (PUFA) supplied in their diets.Mylossoma aureum, a herbivorous piranha, was maintained on oatmeal flakes in which 18:2(n-6) and 18:3(n-3) were the only PUFA and accounted for 40.8 and 1.2%, respectively of dietary fatty acids.Serrasalmus nattereri, the carnivorous red piranha, was fed mosquito larvae containing 26.0-33.4% of their total fatty acids as 18:2(n-6)+18:3(n-3) and 4.9-8.5% as 20:4(n-6)+20:5(n-3). The two species had similar lipid class compositions in liver, brain, viscera and carcass, except that lipids fromM. aureumwere generally richer in triacylglycerols. In both species, visceral and carcass lipid contained high levels of triacylglycerols whose principal PUFA was 18:2(n-6). InM. aureumthe major PUFA in liver total lipid and triacylglycerols was 18:2(n-6) whilst the major PUFA in liver phospholipids were 20:4(n-6) and 22:5(n-6), with 22:6(n-3) being a minor component. The level of 22:6(n-3) in ethanolamine glycerophospholipids was significantly greater in brain than liver ofM. aureum. Although absent from dietary lipid, 22:6(n-3) was the major PUFA in phosphatidylcholine and ethanolamine glycerophospholipids from both the liver and brain ofS. nattereri. In both species, the ratio of (n-6)/(n-3)PUFA was consistently lower in tissue lipids than in dietary lipids. The results are consistent with (i) the herbivorousM. aureumconverting dietary C18 PUFA to their C20 and C22 homologues, (ii) the carnivorousS. nattereriforming 22:6(n-3) from either 18:3(n-3) or 20:5(n-3) and (iii) both species selectively desaturating and elongating (n-3) rather than (n-6) PUFA.