Selenium-enriched exopolysaccharide (Se-ECZ-EPS) produced by bacterium Enterobacter cloacae Z0206 is an important polysaccharide and has been proved its immunostimulating roles in mice. However, relatively less information is available about the potential role of Se-ECZ-EPS in crustaceans. To investigate the modulation of immunity in crustaceans, we examined phenoloxidase (PO) activity, respiratory burst (RB) ability, superoxide dismutase (SOD) level and several immune-related genes' expression profile in red swamp crayfish [ Procambarus clarkii (Girard)] that were individually induced with Se-ECZ-EPS at 1 μg g−1. The protection of crayfish against the white spot syndrome virus (WSSV) by Se-ECZ-EPS was also investigated. The results showed that PO, RB and SOD activities of crayfish increased significantly ( P<0.05) at 24, 48 and 72 h after being injected with Se-ECZ-EPS. Among the genes examined, the mRNA expressions of serine proteinase, HSP70 and Mn-SOD were significantly up-regulated at 24 and 48 h after the Se-ECZ-EPS treatment, while no significant changes in the expression in proPO were observed from 24 to 72 h. The survival rate of crayfish that received Se-ECZ-EPS was significantly higher than that of the control group after 14 days (59.7% and 0% respectively). Taken together, it can be concluded that Se-ECZ-EPS is an efficient immunostimulant and can improve the immunity of crayfish. [ABSTRACT FROM AUTHOR]