Feeding trials were conducted to determine the requirement of Tilapia nilotica for essential fatty acids (EFA) by feeding them with various dietary fatty acids. The fish exhibited poor appetite for the diets containing 12:0 as the basal lipid regardiess of supplement of any fatty acids and consquently the diets resulted in retarded growth and low feed efficiency. The diet containing 18:1ω9 as a sole lipid source gave good growth until 12th week of feeding, but after that the fish began to show poor growth, swollen pale livers and high lipid contents in the lives. The best weight gain were obtained in the fish receiving a diet containing 0.5% or 1% 18:2ω6. Growth enhancing effect of 18:3ω3 was found to be infreior to that of 18:2ω6. Both 20:4ω6 and ω3 HUFA were also found to have no EFA value for fish. Feeding the EFA-deficent diet containing 12:0 or 18:1ω9 as a sole lipid source resulted in raising the level of the “abnormal” polyunsaturated 20:3ω9, and the level was lowered by both ω6 and ω3 fatty acids. The conversion of 18:2ω6 into 20:4ω6 and 22:5ω6 or 18:3ω3 into 22:6ω3 was inhibited to a certain extent by dietary 18:3ω3 or 18:2ω6, respectibely. Thus the dietary requirement of Tilapia nilotica for 18:2ω6 was found to be about 0.5% in diet.