(Uploaded by Plazi for the Bat Literature Project) We describe a new species of mastiff bat (Molossus) from Mexico on the basis of 46 specimens that differ from all other species of Molossus in size, pelage coloration, and morphological characteristics. The new species is most similar to, and specimens were previously identified as, M. sinaloae; but there is no overlap in most skull measurements and the two species differ in development of sagittal and lambdoidal crests and in certain dental characteristics. M. sinaloae occurs in the dry, low tropical deciduous forest of the Pacific versant of Mexico from Sinaloa to Chiapas, a large geographic gap separates it from the new species, which has been found only in the Yucatán Peninsula.