In this study the role of the probiotic protexin in tolerance of larvae of rainbow trout (oncorhynchus mykiss) to physicochemical stresses of water was investigated. In this regard protexin at four different concentrations of 0.15, 0.5, 1 and 1.5 g/kg dried food was used. Larvae with average weights of 2.0±90 mg were fed for 60 days. The experiment was carried out at three replications with 530 larvae in each tray. Feeding varied from 4% to 7% live mass in breeding period. Except larvae of the first treatment, larvae of all other treatments showed higher resistance to heat stress compared with control (p <0.05). Also all treatments survived acidic Ph compared to control treatment (p <0.05). No significant difference in tolerance to alkaline Ph stress was observed between experimental and control treatments (p > 0.05).
