The aim of this study was to examine the immunomodulatory effect of high levels of dietary vitamin C in healthy and immunocompromised rohu (Labeo rohita) treated with aflatoxin B1 (AFB1). Four groups of rohu were fed experimental diets containing either no vitamin C or supplemented with vitamin C at 500 ppm for 60 days. On the first day of feeding, one group fed the high vitamin C diet and one fed the vitamin C deficient diet, were injected intraperitoneally with a single doses of AFB1 at 1.25 mg kg(-1) body weight. The effect of AFB1 and high dietary vitamin C on specific and non-specific immunity, and disease resistance against Aeromonas hydrophila were examined in the rohu. The ability of vitamin C to counteract immunosuppression induced by AFB1 was also examined. Specific immunity indicated by haemagglutination and haemolysin titres against sheep red blood cells (SRBC), and bacterial agglutination appeared to be unaffected by either the AFB1 treatment or the vitamin C enriched diet. A significant reduction was observed in the non-specific immunity of AFB1-treated fish, however, indicated by lowered bactericidal and lysozyme activities. High dietary vitamin C, on the other hand, enhanced the non-specific immunity of fish, including an enhanced phagocytic ratio and increased serum lysozyme activity. Feeding a high level of dietary vitamin C to AFB1-treated fish increased these parameters to levels similar to those found in control fish. High dietary vitamin C significantly (p < 0.05) enhanced protection against Aeromonas hydrophila infection in both healthy and immunocompromised fish. Results from this study help to establish the beneficial effect of dietary vitamin C on AFB1-induced immunosuppression, as well as confirming the immunostimulatory effect of vitamin C in rohu.