A simple, accurate technique has been devised to sex externally channel catfish, Ictalurus punctatus, regardless of body size or state of gonadal maturation. Male catfish have a single urogenital opening; females have two openings partitioned by a septum—an anterior genital pore, and a posterior urinary pore. In male fish, a single probe can be inserted into the urinary bladder through the urogenital pore. Female fish are differentiated from males by the insertion of two probes, one into the urinary bladder through the urinary pore and the second into the oviduct through the genital pore.