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A new species of tuco-tuco, genus Ctenomys, is described from sandy soils on the western slopes of the state of Rio Grande do Sul in southern Brazil. This species is distinguished from other named members of this South American endemic genus by several characteristics. Diagnostic traits for this proposed species are a diploid number of 50 chromosomes and an autosomal fundamental number of 68 arms, with the 1st pair much longer than in other related species. Qualitative and quantitative (geometric morphometrics) analyses of the skull morphology and phylogenetic analysis of the mitochondrial cytochrome-b gene support species status, especially when compared with phylogenetically related and geographically neighboring species. Populations of this species have a narrow geographic distribution in a small area (similar to 500 km(2)) that has been suffering from anthropogenic pressure from soybean, pine, and eucalyptus plantations, as well as desertification. This scenario suggests that this species could be characterized as endangered.