This paper tests Vrba's resource-use hypothesis, which predicts that generalist species have lower specialization and extinction rates than specialists, using the 879 species of South American mammals. We tested several predictions about this hypothesis using the biomic specialization index (BSI) for each species, which is based on its geographical range within different climate-zones. The four predictions tested are: (1) there is a high frequency of species restricted to a single biome, which henceforth are referred to as stenobiomic species, (2) certain clades are more stenobiomic than others, (3) there is a higher proportion of biomic specialists in biomes that underwent through major expansion-contraction alternation due to the glacial-interglacial cycles, (4) certain combinations of inhabited biomes occur more frequently among species than do others.This study was partially supported by the Complutensian University of Madrid (PR1/06-14470-B) and the Spanish Ministry of Education and Science (CGL2004-004000/BTE, CGL2005-03900/BTE, CGL2006-01773/BTE). The Complutensian University of Madrid and the Madrid Autonomous Community provided a grant to the Research Group UCM-CAM 910607 on Evolution of Cenozoic Mammals and Continental Palaeoenvironments, which is directed by Marián Álvarez Sierra (UCM). M.H.F has an UCM research contract from the program "Ramón y Cajal" of the Spanish Ministry of Education and Science.Peer reviewed