We present the results of an inventory of small mammals in the Mayo River basin, one of the least-studied regions of the Central Andes in Peru. We conducted inventories at three locations in May 2007. We collected 47 species of small mammals in the study area: five marsupials, 31 bats, and 11 rodents. A new species of Sturnira was encountered and is described. The new species, which was previously confused with S. lilium, occurs east of the Andes in Colombia, Ecuador, Peru, Bolivia, Venezuela, the Guianas, and Brazil, with an isolated record on the western slope of the Andes in Peru. Additionally, we report the presence of Anoura geoffroyi in Peru, Carollia sp. sensu Solari and Baker (2006) south of the Marañon River, and extend the elevational range of Neacomys spinosus and Oligoryzomys destructor. Our results highlight the need to conduct additional inventories to increase our understanding of the biodiversity of this rich and increasingly impacted region.