A 3 X 2 factorial feeding study was conducted with channel catfish (Ictalurus punctatus) to evaluate effects of biotin, no biotin, or a biotin antagonist (avidin) in lipid and lipid-free diets. At 10 weeks, fish fed diets containing lipid were significantly larger than fish fed lipid-free diets. At 20 weeks, fish fed diets containing avidin had grown significantly less than those fed the other diets. At 22 weeks, fish fed the lipid diet supplemented with biotin had grown significantly more than those fed the lipid diet without biotin. Fish fed the lipid diet with avidin were found to be anemic and exhibited a marked depigmentation of the skin. Fish fed biotin in lipid and lipid-free diets had higher liver pyruvate carboxylase activity than fish fed diets without supplemental biotin. These results indicate that channel catfish require an exogenous source of biotin for maximum rates of growth and lipid utilization.