The Amazon Forest and the Cerrado of central Brazil share an extensive zone of integration. Each of these major ecosystems remains poorly studied; however, the composition of communities at the contact zone is even less well described. Small mammals were sampled during a ten-week period in a zone of contact between Amazonian forest and Cerrado. The vegetation in the area was a complex mixture of tropical and semi-deciduous forest, and grassland and savanna woodland formations. Mammals were marked and recaptured on 0.25- to 0.5-km transects and vouchers were collected. A total of 19 species was caught (13 rodents and 6 marsupials) in 4621 trap nights. Transitions from one vegetation type to another were abrupt with ecotones frequently < 5 m in width. The highest α-diversity was 7 species in cerrado. Ten of the 19 species were restricted to one habitat and 6 were restricted to two habitats. Three generalist grassland species occurred in more than two habitats, all variations of savanna. These data support other research indicating high habitat specificity of vertebrates and plants in the Brazilian savannas. The diversity of habitats within reserves in tropical grasslands must be given consideration in the design of protected areas in order to maximize the conservation of mammalian biodiversity.