Most alternative protein sources to fish meal contain other non-protein energy materials which may influence diet utilization. With the objective of making an overall evaluation of the energy utilization of fish feed ingredients, the following diets were studied: two control diets including either fish meal or casein as the sole protein source (FM and CA100, respectively) and three diets where 40% of the fish meal protein was substituted by one of the following sources: cottonseed meal (CO), lupin seed meal (LU) and corn gluten meal (CG). A sixth diet containing casein (CA40), which replaced 40% of the fish meal protein, was also included as an additional control in order to compare the protein ADC calculation using either fish meal protein or casein protein as reference proteins. All diets were made isocaloric (gross energy) and with the same proportion of estimated available macronutrients. Energy flow studies for dietary protein, fat and carbohydrate, together with an evaluation of the digestive utilization of these energy-yielding nutrients and their efficiency for feed conversion and growth showed that dietary energy efficiency was directly related to the digestible energy of the diets which was especially influenced by the dietary carbohydrate source. Fish adjusted their dietary energy intake to digestible energy levels in the diet. Lupin meal carbohydrate digestibility showed very low values compared to the other plant sources. Protein retention efficiency expressed as protein productive value showed values above 40% for all the experimental diets. Energy retention efficiencies as a function of intake, digestible and metabolizable energy are discussed.