The Siluriformes constitutes an important monophyletic ostariophysan group of mainly freshwater fishes that occurs in almost all continents. They are extremely abundant and diverse in South America, where the oldest otophysans and siluriforms occur. We restudied two putative catfish species from the early Eocene lacustrine Tufolitas Laguna del Hunco of northwestern Patagonia, Argentina, concluding that Arius argentinus Dolgopol, 1941, is a junior synonym of Bachmannia chubutensis Dolgopol, 1941. Phylogenetic analyses were performed to show the relationships of B. chubutensis, which appears as the sister group of Diplomystes. Bachmannia chubutensis is one of the most primitive catfishes known and we based on it a new family. The family is clearly diagnosed by the presence of a anterior ventrolateral expansion of cleithrum, short maxilla with two rows of teeth, double-headed autopalatine, accessory tooth plates, very huge teeth, supraoccipital process long and acute, first and fifth vertebrae not joined to complex vertebra, neural arch of fifth and sixth vertebrae very well developed, enlarged supraneural 5, suture between ceratohyal and epihyal without interdigitations, suture between contralateral cleithra and coracoids without interdigitations, six hypurals. Individuals of B. chubutensis inhabited lakes and were preserved into fine-grained, laminated fall-out and reworked pyroclastic debris inside a caldera.