The work presented in this thesis examined the effects of dietary fish oil replacement on fish innate and adaptive immune function, disease resistance tissue histopathology and fatty acid composition of lipids in peripheral blood leukocytes. Dietary trials with Atlantic salmon (Salmo salar), Sea bass (Dicentrarchus labrax), Atlantic cod (Gadus morhua) and Arctic char (Salvelinus alpinus) were conducted in which fish oil was replaced by rapeseed oil, linseed oil, olive oil, palm oil, echium oil or a mixture of these oils. A significant reduction in respiratory burst activity was most pronounced in salmon and sea bass fed high levels of rapeseed oil-containing diets. In addition, rapeseed and olive oil inclusion in the diets of salmon and sea bass significantly reduced the head kidney macrophage phagocytic capacity to engulf yeast particles. A reduction in prostaglandin E2 levels was found to be related to a reduction in macrophage respiratory burst activity in salmon fed linseed oil diets and sea bass fed a dietary blend of linseed, palm and rapeseed oils. Changes in macrophage function may be a contributing factor causing a reduction in serum lysozyme activity observed in some trials. No significant differences were detected in cumulative mortality of Atlantic salmon fed an equal blend of linseed and rapeseed oils challenged with Aeromonas salmonicida. However, resistance to Vibrio anguillarium was significantly impaired in Atlantic salmon fed a blended oil diet containing linseed, rapeseed and palm oil. The major histological difference of fish fed vegetable oil diets was the accumulation of lipid droplets in their livers. Dietary fatty acid composition significantly affected the fatty acid composition of peripheral blood leukocytes. Generally, fish fed vegetable oil diets had increased levels of oleic acid, linoleic acid and a-linolenic acid and decreased levels of eicosapentaenoic acid, docosahexaenoic acid and a lower n-3/n-6 ratio than fish fed a FO diet. In conclusion, the results from these studies suggest that farmed fish species can be cultured on diets containing vegetable oils as the added oil source. However, feeding high levels of some vegetable oils may significantly alter some immune responses in the fish, especially head kidney macrophage function, disease resistance and, in addition, may cause an increase in tissue histopathology.