The taxonomic status of the species of Necromys Ameghino, 1889 (= Bolomys Thomas, part), inhabiting the pampean region of Argentina and southern Uruguay is not clear. The two most recent systematic hypotheses both distinguish two species. Massoia and Fornes (1967) recognized Necromys obscurus (Waterhouse, 1837) with allopatric populations in southern Uruguay and southeastern Buenos Aires Province (Argentina), and N. benefactus (Thomas, 1919) distributed in a strip across northwestern-southwestern Buenos Aires Province. Reig (1987) argued that the populations of Uruguay and northwestern Buenos Aires Province belong to N. obscurus (including N. benefactus as a subspecies), while those of the southeast and southwest are referred to an unnamed taxon. To solve this problem, 152 specimens from Argentina and Uruguay were studied using morphometric data and qualitative characters. Craniodental variables were measured in 97 adult specimens of both sexes. Discriminant-function analysis among preestablished geographic groups and cluster analysis using Mahalanobis distances revealed the presence of two groups with contrasting morphological characteristics. The first group, which corresponds to N. benefactus, includes the populations from northern La Pampa, southern Santa Fé, and northwestern, centralwest, and southwestern Buenos Aires Province (Argentina). The second group, N, obscurus, includes the populations from Uruguay (N. obscurus obscurus) and from southeastern Buenos Aires Province (a new subspecies).
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