Three diets including 13.2%, 22% or 30.5% carbohydrates as extruded wheat meal were fed to rainbow trout, Oncorhynchus mykiss (initial weight 34.8 g), for twelve weeks to investigate their influence on growth, protein and energy utilization and carcass composition. Daily feeding rates were 1.76, 2.05 or 2.17 g feed per 100 g body weight for the three diets, respectively, so as to reach a daily allowance of about 35.5 kJ digestible energy per 100 g body weight. No significant differences (p>0.05) in final mean weight or specific growth rate were observed between the treatments. Feed efficiency was negatively correlated to the dietary carbohydrate level. Utilized digestible protein and energy per kg growth were 365, 348, 358 g and 15.4, 15.9, 17.0 MJ, respectively, for the three diets. Apparent net protein and energy utilization of the groups fed a medium or high carbohydrate level were higher than those of the low carbohydrate group. Analysis of fish carcasses at the end of the trial revealed no significant (p>0.05) variations attributable to the carbohydrate level.