Freshwater carp and tilapia are the dominant groups of fish produced by aquaculture.Compared with marine fish, the body lipids of freshwater fish generally contain lower levels of nutritionally beneficial long chain n-3 polyunsaturated fatty acids (PUFA), including eicosapentaenoic acid (EPA) and docosahexanoeic acid (DHA). With current interest in the quality and health benefits of fish to consumers, recent attention has focused on the origins and amounts of long chain fatty acids in various farmed freshwater species and on the effects of diet and farming methods on fatty acids content. In the present study, the total lipid and fatty acids content of semi-intensively farmed, Genetically Improved Farmed Tilapia (GIFT) strain (Oreochromis niloticus) was examined and the fatty acids content of both natural and supplemented feeds was determined. Analytical results from commercial feed revealed n-3 PUFA levels of 1.9% of total fatty acids. Plankton samples contained similar total amounts of n-3 PUFA (2.0% of total fatty acids) whilst dorsal muscle (fillet) samples contained 4.8% of total n-3 fatty acids. These results indicate that n-3 PUFA accumulate in muscle lipids. The amounts of EPA and DHA in the dorsal muscle samples were 0.9 and 2.9% of total fatty acids respectively. These values exceed those reported elsewhere for Nile tilapia grown in intensive culture systems with no access to natural food and provide further evidence of the key role of plankton as a dietary source of n-3 PUFA. In a second experiment red hybrid tilapia (Oreochromis sp.) were fed isonitrogenous and isoenergetic diets containing 0, 4, 8, and 12% cod liver oil, substituted against corn oil, with n-3/n-6 ratios of 0.1, 0.2, 0.6 and 1.0 respectively. After feeding triplicate groups, each of 20 fish, for 10 weeks no significant differences were seen in weight gain, specific growth rate, feed conversion ratio, protein efficiency ratio, condition indices, whole body or fillet proximate composition. Significant (P < 0.05) differences were however seen in the fatty acids content of dorsal muscle. There was evidence of selective retention of DHA in the dorsal muscle, where levels significantly exceeded dietary levels.The retention of EPA was low in all dietary groups but was significantly higher in the diet containing 12% cod liver oil. There was no evidence from this study of accumulation of arachidonic acid in the dorsal muscle of treated fish. This has recently been linked to potential production of undesired eicosanoids in some farmed tilapia. The significant decreases in linoleic acid in dorsal muscle samples, as dietary corn oil was reduced, and increase in total n-3 PUFA and DHA contents as dietary cod liver oil was increased, indicate the potential to modify favorably n-3/n-6 ratios of fatty acids in tilapia fillets by dietary manipulation. These ratios increased from 0.1 (fish fed 0% cod liver oil) to 1.44 (fish fed 12% cod liver oil). These results illustrate the contribution of plankton, in the diets of fish farmed in 'green water' systems, to the n-3 PUFA content of tilapia filets. The feeding and growth experiments conducted with red hybrid tilapia provide further evidence of the potential to increase the n-3 PUFA content of tilapia fillets by increasing dietary n-3 PUFA sources.