For developing efficient diets, two sets of experiments examined whether the use and allocation of dietary protein can be traced by labelling with stable isotopes (N-15 and C-13) in two culture fish (Oncorhynchus mykiss and Sparus aurata). In the first experiment, natural abundance and tissue distribution of these isotopes were determined, by measuring the delta C-13 and delta N-15 values by isotopic ratio mass spectrometry, in fingerlings (14-17 g) adapted to diets differing in the percentage of fish meal replacement by plant protein sources. For both species, delta N-15 and delta C-13 were greater in tissues with higher protein and lower lipid content. Delta N-15 of diets and tissues decreased as replacement increased, suggesting delta N-15 can be used as a marker for dietary protein origin. The N-15 fractionation (delta N-15 fish - delta N-15 diet) differed between groups, and could thus be used to indicate protein catabolism. In the second experiment, fish (75-90 g) of each species ingested a diet enriched with N-15-protein (10 g kg(-1) diet) and C-13-protein (30 g kg(-1) diet). These proportions were suitable for determining that the delta values of tissue components were high enough above natural levels to allow protein allocation to be traced at 11 and 24 h after feeding, and revealed clear metabolic differences between species.