RESUMEN
Cnesterodon comprises 10 valid species occurring in the major river basins of South America. Recent ichthyofaunistic studies in the Ivaí River basin, upper Paraná River system, suggested the existence of a possible new species, which was identified as Cnesterodon sp. based on morphological characters. Currently, the use of molecular tools has proved to be fundamental in aiding phylogenetics and cataloging biodiversity; therefore, in this study, we molecularly characterize a possible new species of Cnesterodon from the Ivaí River basin encoding the mitochondrial genes Cytochrome c Oxidase, subunit I (COI), and NADH dehydrogenase subunit 2 (ND2). The genetic differences found showed that this species really differs from the other Cnesterodon species, indicating that it is a distinct species, which is possibly already in serious danger of extinction since its habitat often suffers from human exploitation and its distribution is restricted to only two sites in the upper Ivaí River basin, but it has disappeared in one of them.