This study evaluated dietary polyunsaturated fatty acid (PUFA)/highly unsaturated fatty acid (HUFA) ratios in European sea bass subjected to feeds containing fish oil (FO) or two different vegetable oils (the monounsaturated fatty acid: MUFA-rich rapeseed oil, RO; and the n-6 PUFA-rich cottonseed oil, CSO). Triplicate groups of twenty fish (35 +/- 0.2 g) were fed three fish-based diets in which the added lipid was 100% fish oil (FO), 100% refined low-erucic-acid-rapeseed oil (RO), 100% cottonseed oil (CSO) for a period of 130 days. As a result of this study, final weight, weight gain and specific growth rate were significantly higher in fish fed FO and RO diet compared to fish fed CSO. Final weight in fish fed the RO diet were significantly higher (87.3 +/- 3.1 g) in comparison to fish fed the FO (84.3 +/- 1.2 g) and CSO diets (80.4 +/- 2.3, P<0.05). The concentration of eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA) in flesh was significantly reduced with increasing levels of MUFA and n-6 PUFA in the diet. Accordingly, DHA value in fish fed the CSO diet was 9.41% whereas fish fed the RO diet had lower (7.93%) DHA content in the flesh. As expected, the absorption of individual dietary fatty acids decreased with chain length and increased with the degree of unsaturation. The apparent total lipid digestibility decreased with increasing the PUFA/HUFA ratio in the diet.