Redescription of Diplomystes mesembrinus (Siluriformes, Diplomystidae)

New specimens of Diplomystes mesembrinus collected in the rio Chubut, Argen tina, allow a thorough description of the species, hitherto known only by f our specimens. Diplomystes mesembrinus may be distinguished from D. viedmen sis and D. cuyanus by the possession of a low number of maxillary teeth, no more than 23 in adults. This character is shared by D. mesembrinus and the Chilean species D. chilensis, D. camposensis, and D. nahuelbutaensis. The suture between basibranchials 2 and 3 differentiates D. mesembrinus from th e three Chilean species and D. viedmensis. Some ontogenetic changes in D. m esembrinus are commented upon.