Purified diets with five levels (25, 50, 75, 100, and 200 mg/kg) of supplemental L-ascorbic acid (LAA), and equimolar levels of ethylcellulose coated L-ascorbic acid (EAA) and dipotassium L-ascorbate 2-sulfate dihydrate (AS) were pelleted and fed to 7.9 +/- 0.2 g channel catfish fingerlings for 20 weeks. A dietary level of 23 mg/kg of all three forms of vitamin C prevented spinal abnormalities. Approximately 50 mg/kg diet of either LAA or EAA was sufficient for maximal growth and feed efficiency. Growth response to AS was similar to a Michaelis-Menten type curve and 200 mg/kg diet of AS was necessary to achieve maximal growth. Blood and liver ascorbic acid levels were positively correlated with supplemental levels of LAA, EAA, and AS up to 200 mg/kg; however, blood and liver ascorbic acid levels of fish fed AS were considerably less than those fed LAA and EAA. Weight gains were positively correlated with blood ascorbate levels up to 7 microgram/ml. No measurable level of AS was detected in blood or liver. These results suggest that the rate of enzymatic hydrolysis of AS to LAA or rapid excretion of AS may have been the limiting factor.