Two feeding trials, one under laboratory conditions and the other under practical conditions in a farm, were performed to determine the effect of inclusion of yeast and/or phytase to diets with soybean meal as main protein source, on the growth, P and N excretion, oxygen consumption and serum lysozyme activity of juvenile rainbow trout (Oncorhynchus mykiss). A basal diet (soybean meal, 400 g/kg diet and fish meal, 200 g/kg diet as protein sources) was added either with yeast (Diet Y, 15 g/kg diet), phytase (Diet Ph, 0.4 g/kg diet) and yeast and phyatse (Diet Y+Ph, 15 and 0.4 g/kg diet of yeast and phytase, respectively). A commercial diet was used as a control. Triplicate groups were fed the diets during 70 days (laboratory trial, initial weight 1.2 ± 0.06 g, mean ± standard error) and 75 days (farm trial, initial weight 2.8 ± 0.1 g). At the end of the trials, growth performance, oxygen consumption, P and N excretion and serum lysozyme activity were determined. A trend of higher values of growth was observed in the juvenile fed the diets with soybean meal and yeast (Diets Y and Y+Ph) in both feeding trials. As well, lower values of excretion of P and N were observed in the fish fed Diet Y. Significantly higher value of serum lysozyme activity was observed in the fish fed diet Y under laboratory conditions. Use of diet with 75% of substitution of the fishmeal with soybean meal, added with yeast and/or phytase is reported for the first time to feed rainbow trout under laboratory and farm conditions. The results show the possibility to use such diets, particularly the one with yeast (Diet Y), to fed juveniles without affecting the growth and the immune responses.