Small mammals (Didelphimorphia and Rodentia) from southwestern Buenos Aires Province, Argentina: taxonomy and distribution. Species of small mammals from southwestern Buenos Aires Province range within a wide variety of habitats: herbaceous (Pampean District), shrubs (Monte District), steppes, and xerophytic woodlands (Espinal District). Their natural history is still poorly known. In this paper we make a synthesis of the taxonomy and distribution of the marsupials (Marmosinae) and rodents (Sigmodontinae) of southwestern Buenos Aires Province. Distributional records and taxonomical information were obtained from field survey, analyses of raptor pellets, revision of specimens housed in collections in Argentina, and literature. Between Sierra de la Ventana (38° S) and the Río Negro (41° S) occur two species of marmosines and 14 of sigmodontine rodents. Some of these species show a restricted distribution (e.g., Phyllotis xanthopygus, endemic for Sierra de la Ventana), and others are widespread (e.g.Akodon azarae or Calomys musculinus) across different phytogeographic units. A cluster analysis at local scale shows an ordination of small mammal assemblages with the topology (Sierra de la Ventana + Southern Atlantic Coast) (Espinal + Monte). The zoogeographic unicity of the Monte in the study area is evidenced by two sigmodontines, Akodon iniscatus and Oligoryzomys longicaudatus. The micromammal assemblage of Sierra de la Ventana may be characterized as pampasic. The wide distribution of A. azarae and C. musculinus may reflect the anthropic activities developed during the last century. These activities on shrub steppe have certainly diluted the original zoogeographic boundaries and allowed the dispersion of opportunistic sigmodontine species.