The effects of oral administration of Astragalus polysaccharides (APS) and chitooligosaccharides (COS), single or combined, on the growth performance, immunity and disease resistance of M. salmoides were investigated. Largemouth bass juvenile were divided into 4 groups and each group was fed with diets supplemented with or without immunostimulant for 8 weeks. After 8 weeks of feeding trial, five fish per tank were sampled for immunity determination, ten fish per tank were challenged by A. hydrophila. The results showed that the largemouth bass fed with two immunostimulants alone or in combination significantly enhanced the final weight and specific growth rate (SGR), decreased feed conversion ratio (FCR) (P < 0.05). However, there were no significant differences (P < 0.05) in specific growth rate (SGR) between dietary COS and dietary APS. In addition, both COS and APS upregulated respiratory burst activity (RBA), phagocytic activity (PA), lysozyme activity and superoxide dismutase (SOD) activity. Meanwhile, COS also exhibited a increase in total leukocyte count, nitric oxide (NO) content and inducible nitric oxide synthase (iNOS) activity compared to the control. When challenged with A. hydrophila, the mortality of groups fed with COS and/or APS was lower than the control (P < 0.05). Under the experimental conditions, dietary APS and COS had a synergistic effect on lysozme activity, iNOS activity, NO content and disease resistance of fish (P < 0.05).