The aim of this study was to investigate whether fish size has an effect on the utilisation efficiency of digestible protein, digestible fat and digestible carbohydrates (Carb) for energy gain in rainbow trout. Four different diets ranging in protein, fat and Carb were fed to two size classes of rainbow trout, 30 to 75 g and 92 to 214 g, at three different feeding levels (12, 8 and 4 g/kg