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°. 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 semipurified diets, yet no incidences of spinal abnormalities were noted.