Chilean Mediterranean ecosystems, the only of this type present in South America of the four present in the world, are considered priority areas for conservation due to their high concentration of endemic species that have experienced accelerated rates of habitat destruction. They contain more than 39% of the mammal species, 47% of its endemic species, and 65% of the threatened species of Chile. Yet, these ecosystems are poorly represented in the system of protected areas, one of which is the Reserva Nacional Lago Peñuelas (RNLP) that is part of the Biosphere Reserve La Campana-Peñuelas, but whose mammal fauna is poorly documented. We studied both α and β diversity of the mammal assemblage in all three environments present at the RNLP (sclerophyllous forest, mixed shrub, and savanna of Acacia caven). Sherman traps grids were installed, pellets of two raptors (Tyto alba and Bubo magellanicus) were analyzed, tracks and signs were recorded, and direct observations were performed in the four seasons of the year 2001. We determined species richness (S), relative abundance, α diversity - considering its richness and structure (Shannon-Wiener and Pielou indexes)-, β diversity (Bray- Curtis index), and compared the diversity found with that documented for the same latitude from east to west. We recorded a total of 16 species: Thylamys elegans, Oligoryzomys longicaudatus, Abrothrix longipilis, A. olivaceus, Chelemys megalonyx, Phyllotis darwini, Myocastor coypus, Octodon degus, O. lunatus, Spalacopus cyanus, Abrocoma bennetti, Rattus norvegicus, R. rattus, Mus musculus, Lepus capensis, and Oryctolagus cuniculus. The coastal sclerophyllous forest was the most diverse with a species homogeneous distribution, followed by the mixed shrub, and finally the A. caven savanna. Also, the sclerophyllous forest is similar to the mixed scrub; in turn, both are very dissimilar to the savanna of A. caven. The diversity recorded in the study area is consistent with that of other areas of the Mediterranean areas. We discuss the status of these ecosystems and the diversity is compared to six documented locations at the same latitude.