Mitochondrial cytochrome-b DNA and karyologic data were used to clarify taxonomic status of genera Coendou and Sphiggurus within the family Erethizontidae (Rodentia: Hystricognathi). Molecular and karyologic data were concordant in demonstrating that Coendou and Sphiggurus represent 2 evolutionary lineages. Comparative analyses showed that species of Coendou are karyotypically conservative, sharing the same diploid and fundamental number, whereas those of the genus Sphiggurus diverge in diploid number despite sharing the same fundamental number. We herein describe the karyotype of Sphiggurus melanura, a species considered a junior synonym of S. insidiosus by some authors. Our karyologic data (2n = 72, fundamental number = 76), however, clearly contradict this hypothesis and suggest that S. melanura is a valid taxon.