The aim of the present study was to determine the effects of replacing fish oil (FO) with cottonseed oil (CSO), as an alternative plant lipid source in diets for rainbow trout (89.3 +/- 1.1 g mean individual weight), on the growth and fatty acid composition of fish. As a control diet, 5 experimental diets were formulated with pure FO and with partial or complete (25%, 50%, 75%, or 100% CSO, respectively) replacement of FO with CSO. The fish were fed 2% of their body weight per day for 60 days at an average water temperature of 10.2 degrees C. At the end of the feeding trial, the highest growth and the lowest FCR (1.28) were obtained in fish fed the CSO50 diet. The fillet lipid concentrations and fatty acid composition of the fish were significantly affected by the experimental diets. Fish fed the FO and CSO25 diets contained significantly lower lipid levels (6.8% and 6.5%, respectively) than those fed the 3 other diets. The viscerosomatic index (VSI) and hepatosomatic index (HSI) values increased with increasing cottonseed oil percentages in the diets. Fillet fatty acid composition reflected dietary fatty acid composition. The n-6 polyunsaturated fatty acid (PUFA) concentration increased with increasing cottonseed oil levels in the diets. In contrast, the n-3 PUFA levels decreased with increasing cottonseed oil levels in the diets. The highest level of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) concentrations were recorded in fish fed the FO diet and the lowest in those fed the CSO100 diet. Based on the results of growth performance and fatty acid composition of the experimental fish in the present study, it can be concluded that the 50% concentration of cottonseed oil performed best among the diets tested in the experiment.