Rainbow trout (Oncorhynchus mykiss) were fed three different diets for 110 days -a basal dry diet with 8.4% oil content (BD8), a basal dry diet with 11.1% oil content (BD11) and an expanded diet with 20.7% oil content (ED) -to investigate the influence of high fish oil expanded diet on fatty acid composition of muscle, and to evaluate nutritional properties of edible tissue. In fact, the experimental diets were also different in their component fatty acids, with an increasing content of oJ3 highly unsaturated fatty acids (co3 HUFA) from BD8 to ED. As regards biometrics data, the condition factor and the coefficient of fatness were higher in fish fed ED in comparison with groups BD8 and BDll (p < 0.05 ED vs. BD8). On the other hand, hepatosomatic index in group ED was markedly lower than those in groups BD8 and BD11 (p < 0.05 ED vs. BD8 and ED vs. BD11). This could be explained by the lower amount of crude protein in ED or it may indicate an excess amount of essential fatty acids (EFA) in ED. As regards fatty acid composition of fish muscle, there were only slight differences in fatty acid composition of the edible tissue of fish when compared with the differences in fatty acid composition of the diets. The increased amount of fish oil in ED had a positive influence on the final weight of fish (p < 0.05 ED vs. BD8 and ED vs. BD11), but did not affect proportionately the percentage of ¢o3 HUFA (20:50)3, 22:5co3, 22:6o~3) and therefore the derived indices of lipid quality: so it appears possible to partially substitute fish oil in the diet with other lipid as a source of dietary fat.