Oedemognathus exodon, a monotypic genus and species of Amazonian apteronotid electric fishes, is based on a single specimen with uniquely externalized jaw teeth projecting from a bulbous snout. Recently, many additional specimens of Oedemognathus, all mature males, have been collected in the Amazon by trawling in channels and seining in floating vegetation mats. These new specimens were collected with many monomorphic female, male and immature specimens of Sternarchogiton nattereri, a related and similar species. On this basis we suggested a hypothesis of sexual dimorphism and conspecificity for O. exodon and S. nattereri. Although this idea has been taxonomically accepted by recent authors with treatment of O. exodon as a junior synonym of S. nattereri, the evidentiary basis for this remains unpublished. Herein we present that evidence. In addition to being associated at sexual maturity during breeding season, S. nattereri and O. exodon are closely similar in external and internal morphology, pigmentation, and mitochondrial cytochrome b sequences. Furthermore, we observe that the striking dentition, jaw and snout morphology of Oedemognathus has its developmental origin in mature males that are equal, or nearly so, in size to non-dimorphic S. nattereri.