In this study, body composition and digestive enzymes activity was assessed in rainbow trout fry (weight 2.51 ± 0.55 g) fed by bacterial single cell (BSC) as a dietary protein source replaced with fish meal as completely randomized design with triplicate for 6 weeks. Experimental diets were prepared in 5 treatments as isonitrogenous with different levels of fish meal and BSC (100% fish meal (A), 75% fish meal and 25% BSC (B), 50% fish meal and 50% BSC (C), 25% fish meal and 75% BSC (D) and 100% BSC (E)). The body composition findings showed that the highest amount of lipid and protein and the lowest amount of moisture and ash was revealed in fish fed by diet C while the lowest amount of lipid and protein and the highest amount of moisture and ash was observed in fish fed with diet E (P <0.05). Enzymatic assessment from pyloric caeca and intestine were showed that trypsin and α-amylase were increased significantly in fish fed by diet C while alkaline phosphatase activity had the highest amount in diet D than did that from diet E (P <0.05). Based on the results of this study, it is recommended that BSC could be properly replaced up to 50 % with fish meal in rainbow trout fry diet.