We report the first record of Molossus aztecus from two localities in southeastern Brazil, Lavras and Viçosa, confirming the presence of this species in South America; both localities are located in the state of Minas Gerais. Samples from Lavras contained both M. aztecus and M. molossus, permitting direct comparison of the two taxa. At both sites, the original vegetation was Cerrado-Atlantic Forest ecotone and the bats were captured at elevations from 650 to 1,100 m. We compared our specimens of M. aztecus from southeastern Brazil with M. aztecus from Mexico, M. molossus from several parts of Brazil, M. coibensis from Panama, M. barnesi from French Guiana, and M. currentium from Paraguay. Capturing M. aztecus and M. molossus sympatrically reinforced their distinctiveness; they differ in morphometrical traits and qualitative characters of skull and pelage. We also found that specimens earlier identified as M. coibensis from Brazil, are referable to M. barnesi from French Guyana.