Three feeding studies on the vitamin C requirements of channel catfish were conducted with practical and semipurified diets. In a long-duration study in which fish achieved almost a 4,000% increase in weight, 50 mg of l-ascorbic acid/kg diet was required for maximal growth and food efficiency. A diet stability study revealed that excessive losses in activity of l-ascorbic acid occurred when practical diets were stored for 16 weeks at 20 degrees. The typical scoliosis condition associated with severe vitamin C deficiency in fish occurred in the nonsupplemented groups in the study with practical diets. Severe growth reductions were obtained from fish fed nonsupplemented semi-purified diets, yet no incidences of spinal abnormalities were noted.