Isoelectric focusing (IEF) and densitometric scanning were used to identify 6 species of catfish based on diagnostic bands and banding patterns produced by muscle proteins. Blue catfish (lctalurus furcatus), white catfish (Ictalurus catus), and flathead catfish (Pylodictis olivaris) each have diagnostic bands in pH 4-5 gels that allow positive species identification. IEF of channel catfish (Ictalurus punctatus) musculature in this pH range exhibits a polymorphic system. One of the bands is diagnostic for this species when present, allowing identification of 58% of the channel catfish tested. Channel catfish without this band, yellow bullhead catfish (lctalurus natalis), and black bullhead catfish (Ictalurus melas) are indistinguishable in the pH 4-5 range gels, but can be identified by banding patterns produced by IEF of proteins in pH 6-7 gels.