This study aimed to investigate the effects of complementary rainbow trout diets with different concentrations of Celmanax® (active compounds of Saccharomyces cerevisiae with mannan-oligosaccharide (MOS)) on immune responses, pressure resistance and resistance to Yersiniosis. Rainbow trout were fed with a diet containing various concentration levels of S.cerevisiae with MOS (prebiotic) (0, 0.1, 0.5 and 1%) for 60 days. While evaluating some of the parameters of the immune system, the blood samples were prepared from the tuber stem vein every 30 days. On day sixty of the study, various stress tests including temperature increases, hypoxia and induction of experimental disease with Yersinia ruckeri were also performed in all the experimental groups. Results showed that lysozyme activity, alternative complement pathway and total antibody were significantly elevated by diets containing different concentrations of prebiotic and the effective supplementation diet concentration was found to be 0.1% (p<0.05). However, the results of environmental pressures and exposure to bacteria showed that rainbow trout resistance was increased with different concentrations of prebiotic and the effective supplementation diet concentration was also 0.1% (p<0.05). Based on these findings, it is suggested that to increase and enhance immunity and improve rainbow trout resistance, it would be appropriate to add 0.1% concentration of prebiotic in the diet.