Effects of irradiated and non-irradiated Ergosan extract(alginic acid) on rainbow trout growth performance and skin mucosal immunity were compared. Ergosan was irradiated at 30 kGy in a cobalt-60 irradiator. A total of 252 fish(128.03±9.4 g) were randomly divided into four equal groups,given the basal diet either unsupplemented with Ergosan(control group) or supplemented with crude Ergosan(5 g/kg),ethanol-extracted Ergosan(0.33 g/kg) or irradiated Ergosan(0.33 g/kg) according to this protocol: basal diet for 15 days,treatment diet for 15 days,basal diet for 10 days and treatment diet for 15 days. Highest growth performance was observed in fish fed irradiated Ergosan(P<0.05). Dietary administration of different Ergosan types did not cause any changes in mucus protein level,but improved alkaline phosphatase level and hemagglutination titer compared with the control(basal diet without Ergosan) on day 55 offeeding trial(P<0.05). Furthermore,the highest value of lysozyme activity was observed in gamma-irradiated Ergosan on day 55. In conclusion,gamma-irradiated Ergosan at 0.33 g/kg was found to improve growth performance and mucus biological components significantly in comparison with the control group(basal diet without Ergosan). Effects of irradiated and non-irradiated Ergosan extract (alginic acid) on rainbow trout growth performance and skin mucosal immunity were. Ergosan was irradiated at 30 kGy in a cobalt-60 irradiator. A total of 252 fish (128.03 ± 9.4 g) were randomly divided into four equal groups, given the basal diet either unsupplemented with Ergosan (control group) or supplemented with crude Ergosan (5 g / kg), ethanol-extracted Ergosan (0.33 g / kg) or irradiated Ergosan according to this protocol: basal diet for 15 days, treatment diet for 15 days, basal diet for 10 days and treatment diet for 15 days. Highest growth performance was observed in fish fed irradiated Ergosan (P <0.05). Dietary administration of different Ergosan types did not cause any changes in mucus protein level, but better alkaline phosphatase level and hemagglutination titer compared with the control (basal diet without Ergosan) on day 55 offeeding trial (P <0.05). Furthermore, the highest value of lysozyme activity was obser ved in gamma-irradiated Ergosan on day 55. In conclusion, gamma-irradiated Ergosan at 0.33 g / kg was found to improve growth performance and mucus biological components significantly in comparison with the control group (basal diet without Ergosan).