AbstractThe gross morphology of the gas bladder is described and illustrated for representatives of most species and all valid genera of the Auchenipteridae (Siluriformes). Although, a simple cordiform gas bladder is present in some species of the family, others are characterized by their distinctive gas‐bladder shape and diverticula disposition. An acute posterior end of the gas bladder characterizesCentromochlus heckeliiandC. macracanthus, and is variably present in specimens ofAuchenipterus.Tocantinsia piresiandAsterophysus batrachushave distinctive gas bladders differing in number of diverticula (two or many). The two species ofTrachycorystesare diagnosed based on their gas bladder morphology:T. menezesihas a simple cordiform bladder, whereasT. trachycorysteshas a pair of lateral diverticulum and, usually, a well‐developed terminal diverticulum. Species ofAuchenipterichthysare characterized by having a secondary bladder with simple chamber. Short or elongate and divergent terminal diverticula are exclusive to most cis‐andine species ofTrachelyopterus.Tetranematichthysand trans‐andine species ofTrachelyopterusshare a well‐developed secondary chamber or terminal diverticula ventrally or dorsally connected to the posterior chambers. The small‐sized species ofAgeneiosushave well‐developed gas bladders with a pair of posterior diverticula, whereas large‐sized species have a reduced gas bladder with tunica externa varying from non‐, partially, or completely ossified. Eight phylogenetic characters are proposed based on the morphology of the gas bladder and associated structures in species of Auchenipteridae, and the evolution of those characters in the family discussed. J. Morphol., 2012. © 2012 Wiley Periodicals, Inc.