This study aims to evaluate the effects of Bifidobacterium animalis PTCC-1631 and Bifidobacterium lactis PTCC-1736 on growth, nutrient digestibility, carcass composition, and intestinal microbiota of Oncorhynchus mykiss fry, which had initial body weights of 0.58 ± 0.19 g. Four iso-nitrogenous diets with probiotics, namely T₁ (1 × 10⁷ CFU g⁻¹), T₂ (2 × 10⁷ CFU g⁻¹), T₃ (3 × 10⁷ CFU g⁻¹), and control without probiotics were obtained. Total 480 fish were randomly divided in four treatments (three experimental treatments and a control), each with 4 replicates including 16 tanks each filled with 15 L⁻¹ of water (water exchange rate of 2 L min⁻¹ and aerated with air stone) with a density of 2 fry L⁻¹. The water mean temperature was 17.66 ± 1.33 °C, and the mean pH value was 7.63 ± 0.08. After eight weeks of the experiment T₁ that fed with a diet contained the lowest bacterial concentration showed higher growth, nutrient utilization, digestibility, and lower feed conversion ratio (P < 0.05) in comparison with T2, T3, and the control. Also, the highest count of lactobacillus bacteria was observed in T₁ that was fed with a diet containing the lowest probiotic. The lowest concentration of Bifidobacterium strains in this study induced better growth and increased digestion and nutrient utilization in trout fry.