A study was conducted to compare utilization of ME for growth vs. maintenance in rainbow trout and Atlantic salmon. Fish were hand-fed to satiation one of four isoenergetic diets (DE = 20 MJ/kg, as-fed basis) with different digestible protein (DP) to DE ratios (24, 22, 20, and 18 g/MJ). Intake of ME (kJ/d), energy deposited as protein (PD, kJ/d), and energy deposited as lipid (LD, kJ/d) were determined by a comparative slaughter technique. Data were analyzed by a factorial approach or by multivariate analysis of PD and LD on ME. Maintenance energy requirements (ME(m)) and efficiency of ME utilization for PD (k(p)) and LD (k(f)) were estimated with both approaches. For the multivariate analysis, an additional parameter, the fraction of ME intake above maintenance used for PD (X) was defined as linear function of BW, with slope (d) and intercept (c) estimated simultaneously with the above parameters. Estimates were highly dependent on the approach and assumptions used. The ME(m) and k(p) values were higher and less accurate with the factorial approach than with multivariate analysis. The factorial approach estimated unrealistic k(f) values (k(f) > 1). With the multivariate analysis, ME(m) did not differ between species (20 kJ x d(-1) x kg(-0.8)). On the other hand, k(p) was significantly higher (e.g., 0.52 +/- 0.06 vs. 0.43 +/- 0.06; P < 0.05) for salmon than for trout and independent of diet, but k(f) was 0.81 (+/-0.13) regardless of species or diet. The ME intake above ME(m) used for PD (c) was higher in salmon than trout (57 vs. 55%; P < 0.05). The change in partitioning of ME for PD due to the change in BW was negative for trout (d = -0.18), but positive for salmon (d = 0.16). The d values agreed well with the increase of LD:PD ratio with BW for trout and the decrease of LD:PD with BW for salmon, which may have been related to the maturation status of this fish and the associated loss of body lipid observed by maturing salmon. In conclusion, ME(m) and cost of LD were similar for rainbow trout and Atlantic salmon, but the cost of PD was lower for salmon than for trout.