The aim of this work was to study the fatty acid (FA) bioconversion ability in Eurasian perch fed with diets differing in their polyunsaturated fatty acids (PUFA) from n-3 and n-6 series content at two development stages: adults in exogenous vitellogenesis, and juveniles during the on-growing phase. Duplicate groups of adults and juveniles were fed for 12weeks with four diets: D1 and D2, two diets prepared with fish oil partially or totally as the lipid source, and so containing long-chain PUFA (LC-PUFA). Those two diets differed by their n-3/n-6 FA dietary ratio (0.2 and 7.0, respectively), D1 being characterized by a high n-6 LC-PUFA level, while D2 had a high level of n-3 LC-PUFA. D3 and D4 were constituted only with vegetable oils, and were therefore devoid of LC-PUFA. D3 was characterized by a high level of 18:2 n-6 (n-6/n-3 ratio of 0.3), while D4 was characterized by a high level of 18:3 n-3 (n-3/n-6 ratio of 1.9). Both groups of fish were able to elongate and desaturate the 18:3 n-3 precursor into eicosapentaenoic acid and docosahexaenoic acid, regarding the FA profile of livers. Furthermore, total elongation/desaturation from [1- C]18:3 n-3 of LC-PUFA was higher in fish fed with the high dietary 18:3 n-3 level compared to the diet rich in n-3 LC-PUFA. By opposition, the bioconversion of 18:2 n-6 into LC-PUFA was limited, regarding the elongation/desaturation activity of LC-PUFA from [1- C]18:2 n-6. In view of the great ability for bioconversion of n-3 FA, linseed oil is a promising alternative to fish oil in formulating feed for juveniles perch as there were no differences in terms of specific growth rate between the treatments, but adults undergoing maturation should have at least partially LC-PUFA in their diet, particularly arachidonic acid (ARA) which is important during maturation, as breeders are not able to bioconvert 18:2 n-6 into ARA.