James S. Albert and William L. Fink

A new species of gymnotoid knifefish in the genus Sternopygus is described from the Rio Xingú of Brazil. A data matrix of 52 characters for the 4 recognized species of Sternopygus and representative outgroup taxa is presented. The data include 12 characters diagnostic of the Sternopygidae, 11 characters supporting the monophyly of Sternopygus, and 15 characters structuring a hypothesis of relationships among the four species of Sternopygus. Variation among and between populations of Sternopygus from many localities in South America remains to be analyzed for a robust hypothesis about numbers of species and their interrelationships. Characters of the genus thought to be plesiomorphic within the Gymnotoidei are discussed in relation to the proposed phylogeny. Implications of this phylogeny on the biogeographic history of Sternopygus are noted. A key to the recognized species of Sternopygus is provided.