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. The fur is soft and somewhat long (5mm). The dorsal side is gray-brown or brown whereas the ventral side is gray-brown. Little else is known specifically about this species but many physical traits are common to all Eumops species. Each species in this genus have a thick head with a broad muzzle. The eyes of Eumops are small compared to species of bats from other genera. Eumops have legs which are short, strong, and muscular with broad feet and a well developed fibula. The wings of Eumops are long, flat, and narrow giving them a high-aspect ratio. The dental formula for the genus is I:1/2,C:1/1,P:2/2, and M:3/3. Measurements (in mm) specifically for E. bonariensis are: head-body length 49-68, tail 28-47, hind foot 6-11, ear 12-19, and forearm 39-48. (Anderson and Jones 1984, Emmons and Feer 1990, Eisenburg 1989, Kunz 1982, Nowak 1999, Redford and Eisenburg 1992, Reid 1997)
Range mass: 7 to 13 g.