By means of a comparative slaughter technique, energy balances were determined in growing rainbow trout. Seven parts of a fish meal/casein basal mixture were blended with either 1.3 parts sunflower oil or 3 parts of one of the following compounds: glucose, sucrose, lactose, gelatinized maize starch or gelatine. Each of these mixtures was pelleted with an adequate amount of the basal mixture. Equivalent amounts of the mixtures, containing one of the carbohydrates or gelatine, and the basal mixture were fed to duplicate tanks containing 11 trout of 15 g initial live weight. In addition, the mixture containing sunflower oil was fed at 83% and the basal mixture at 70% of this amount to duplicate tanks. The trials lasted for 46 and 47 days. Thirty-one to 46% of dietary gross energy, and 21–38% of dietary nitrogen were retained. Efficiencies of utilization of gross energy can be calculated from the differences in the results between each supplemented group and the group fed the basal mixture at the low level for the following supplements: lactose 11.0%, gelatine 19.4%, fish meal/casein 43.1%, glucose 57.4%, gelatinized starch 61.2%, sunjlower oil 63.2%, sucrose 63.5%. With the exception of sunflower oil, all supplements caused increased concentrations of ether extract and N-free extract in trout livers.