Abrothrix illutea Thomas, 1925 is a poorly known sigmodontine rodent of the recently diagnosed tribe Abrotrichini. Here we report new locality records in northwestern Argentina, describe in detail several morphological traits of this mouse and provide an emended diagnosis for the species. Abrothrix illutea is characterized by a unique combination of characters, including uniform dark olive-grey dorsal coloration; skull robust, with the interorbital constriction hourglass shaped and smoothly rounded; muzzle elongate; nasals and premaxillae slightly projected anterior to the incisors; palate ending at about the plane of the last upper molars; upper incisors broad and orthodont; molars hypsodont, M1 with anteroloph and mesoloph well developed; stomach unilocular-hemiglandular; terminal crater of the phallus ventrally directed; cartilaginous distal baculum reduced, composed of two lateral digits. These morphological features are discussed at the light of phylogenetic analyses of nuclear and mitochondrial DNA sequences. Results of these analyses, the first to include A. illutea, are incongruent between loci. It is suggested that the nuclear based tree better reflects the pattern of morphological variation, and presumably the species tree, than the mitochondrial tree. These aspects are discussed in reference to the contents and limits of the genus Abrothrix. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.