Channel catfish Ictalurus punctatus fingerlings in ponds were fed diets containing 0, 100, 500, 1000 or 4000 mg of vitamin C kg-' of diet for 9 wk. They were then immunized with EdwardsieUa jctaluri. One month after immunization, they were challenged with virulent E. ~ctaluri. Fish showed no clinical signs of vitamin C deficiency during the experimental period and final weights of the fish on the vanous diets were not significantly different ( p > 0.05). Increasing the level of vitamin C in the diet did not affect complement hemolytic activity and antibody titers. However, fish receiving vitamin C at 1000 mg kg-' showed increased resistance to E. ictaluri. These fish exhibited a 100-fold LDS0 compared to fish receiving a diet without supplemented vitamin C.