This study was conducted to evaluate the effects of Alphamune, a mixed prebiotic composed of mannan-oligosaccharide and β-glucans, on the growth performance, digestibility and enzyme activity of rainbow trout, Oncorhynchus mykiss. A basal diet was formulated using common feed ingredients supplemented with Alphamune at 0, 0.5, 1, 2, and 4 g.kg^-1 leading to five experimental diets. Obtained results showed that inclusion of dietary Alphamune significantly increased the final weight and weight gain (p<0.01) of rainbow trout compared to the control group. Feed conversion ratio was also improved after prebiotic administration in comparison with the control group (p<0.05). However, Alphamune supplementation did not change specific growth rate (p>0.05). Also apparent digestibility coefficient (ADC) was not affected by dietary addition of the prebiotic (p>0.05). Our results indicated that amlayse and lipase activities were not significantly influenced by administration of different doses of Alphamune. The result also showed that trypsin activity was gradually increased with increasing of the probiotic level (p>0.05). In conclusion, inclusion of the prebiotic Alphamune can improve the nutrient efficiency and growth performance of rainbow trout confirming the positive effect of a mixture of prebiotics on fish.