All‐male populations of channel catfish, Ictalurus punelatus, were produced by feeding a diet containing trenbolone acetate to swim‐up fry for 60 d. This hormone was effective in producing 100% males at doses of 50 to 150 mg/kg of diet. Fish that received 0 or 1 mg/kg of diet did not differ from the expected 1:1 sex ratio. Survival of control fish and fish fed diets containing trenbolone acetate were similar (P > 0.05) at the end of the treatment period and after a 60‐d grow‐out period. Our results represent the first report of successful masculinization of channel catfish with a synthetic anabolic androgenic steroid.