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In a completely randomized design, the effects of two probiotic bacteria, Bifidobacterium animalis and B. lactis at four concentrations of 0 (control), 1×10⁹ (T1), 2×10⁹ (T2) and 3×10⁹ (T3) CFU 100g^-1 on nutritional parameters of rainbow trout fry (0.538 ± 0.197 g body weight) were compared for 60 days, with each treatment carried in three replications. A significant difference was observed between treatments and the control in protein and lipid gain and also body composition (p<0.05). Despite a significant difference of metabolic growth rate in T1 and T2 with T3 and the control, no significant difference was observed between T3 and the control. The results demonstrated that the use of thesebacteria could improve nutritional parameters and metabolic growth rate of rainbow trout fry.