The fatty acid composition of the pejerrey Odontesthes bonariensis larvae was studied to elucidate potential dietary relationships. Their principal fatty acids were characteristic for membrane lipids of aquatic organisms. The fatty acid composition varied little throughout seasons, with high proportions of 22: 6(n-3) (27% of total fatty acids), which is biosynthesized de novo from dietary precursor fatty acids and/or accumulated from the diet. Other major fatty acids were 16: 0 and 18: 0. The diatom-typical 16: 1(n-7) and other dietary fatty acids (zooplankton and microplankton) are not reflected in the larvae, thus limiting the use of fatty acids as trophic markers for food web relationships of atherinopsids.