The range of the mastiff bat, or Eumops bonariensis , extends from Veracruz, Mexico, through Central America and into South America. In southern South America mastiff bats are found throughout Paraguay, sections of Uruguay, and as far south as the Buenos Aires province in Argentina. Mastiff bats have only been found in the lowlands throughout this range. Eumops bonariensis is the smallest of the eight species which comprise the genus Eumops .
Mastiff bats have agouti hair coloring.
The base of each hair is pale while the tips are frosted. Bristles are not located on the rump but some short hairs can be found on the calcar, which is the bone that gives structure to the membrane between the tail and the hind foot. The snout is broad and flat with a few slight wrinkles on the lips. Their large ears are rounded and, when laid forward, touch the tip of the nose. Little else is known specifically about this species but many physical traits are common to all Eumops species.