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 Maranon 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. VELAZCO AND PATTERSON: SMALL MAMMALS OL THE MAYO RIVER BASIN, PERU 7 Condyloincisive length (CIL): From the posteriormost point on the occipital condyles to the anteriormost point on the upper incisors. Condylocanine length (CCL): From the posteriormost point on the occipital condyles to the anteriormost point on the upper canines. Lacrimal breadth (LB): Greatest breadth across the lacrimal (= anteorbital) ridges, when present and well defined. Postorbital breadth (POB): Least breadth across the frontals posterior to the postorbital pro¬ cesses or bulges. Zygomatic breadth (ZB): Greatest breadth across the zygomatic arches. Braincase breadth (BCB): Greatest breadth of the globular part of the braincase. Mastoid breadth (MB): Greatest cranial breadth across the mastoid region. Maxillary toothrow length (MTR): From the ante¬ riormost edge of the canine crown to the posteriormost edge of the crown of M3. Breadth across molars (BM): Greatest breadth across the outer edges of the crowns of the upper molars. Breadth across canines (BC): Greatest breadth across the outer edges of the crowns of the upper canines. Dentary length (DENL): Distance from midpoint of condyle to the anteriormost point of dentary. Mandibular toothrow length (MANDL): Distance from anteriormost surface of the lower canine to the posteriormost surface of m3.We follow Davalos et al. (2014; MorphoBank project 891, http://dx.doi.org/10.7934/P891) in assigning homology for bat premolars. When three upper premolars are present (e.g., Anoura), the first upper premolar corresponds to PI, the second to P4, and the third one to P5. If only two are present (e.g., Lophostoma) these correspond to P4 and P5 and if only one is present (e.g., Desmodus) it corresponds to P5. The lower premolar homologies are not as straightforward as those of the upper premolars. When three lower premo¬ lars are present (e.g., Glossophaga, Thyroptera)these correspond to pi, p4 and p5, but if only two are present in some cases they apparently corre¬ spond to pi and p5 (e.g., Artibeus, Sturnira, Uroderma) whereas in other cases ...