The southernmost limit of the distribution of endemic Andean cutaneous leishmaniasis (CL), commonly known as Uta, is localized in the western Andean valleys of Ayacucho, Peru. This area is completely isolated from other regions endemic for this disease. Identification of the insect vector for Andean CL was carried out by combining entomologic and parasitologic approaches. Two Lutzomyia species were captured: Lutzomyia ayacuchensis and Lu. noguchii. The former species was considered responsible for transmission of Leishmania because 1) there was a coincidence in space and time between the presence of this insect and the distribution of Andean CL, 2) it was shown to be highly anthropophilic, 3) Leishmania parasites of the subgenus Viannia were detected by a specific polymerase chain reaction assay, 4) promastigotes isolated from this insect were shown by multilocus enzyme electrophoresis and molecular karyotyping to belong to the same deme of Leishmania (Viannia) peruviana as the one circulating in humans living in the study area, and 5) the complete cycle of L. (V.) peruviana was observed in experimental infections of Lu. ayacuchensis. Parasite and vector homogeneity found in Ayacucho contrasted with the heterogeneity reported for other areas endemic for Andean CL. The potential influence of ecologic determinants on this geographically isolated area is discussed.