Sixteen hundred channel catfish fingerlings were stocked in 1-mE3 floating cages, 400 per cage, and fed production-type diets intensively for 180 days. Half of the fish were fed a diet that was nutritionally sufficient except for being low in vitamin C and the remaining fish were fed the same formulation with supplemental vitamin C. At the end of the feeding period each fish was examined for gross signs of vitamin C deficiency and fish were sampled for examination for internal symptoms by X-ray, tissue analysis, and histological techniques. Weight gains (g), feed conversions (g gain/g feed), survival (%) and incidence of physical deformation (%) for fish fed diets with and without supplemental ascorbic acid, respectively, were 531 vs. 331; 1.29 vs. 1.88; 98.0 vs. 77.8; and 3.9 vs. 45.0. Deformities were primarily scoliosis and lordosis with depigmented areas on backs. Vitamin C deficiency resulted in higher mortality from pathogenic bacteria infestation. Differences in response to the two diets did not become apparent until after the fish had grown satisfactorily for at least 12 weeks or weighed 100 to 180 g. Dietary supplementation of vitamin C was reflected in blood, liver and anterior kidney contents of ascorbic acid but not in condition of gill filaments or opercula.