Infant babbling in humans and a few other primates plays an important role in allowing the young to practice the adult vocal repertoire during early behavioral development. Vocalizations uttered during babbling resemble, to some degree, the acoustic structure of adult vocalizations and are often produced in long bouts independent of any social context. Similar behavior, termed subsong or plastic song, is known from a variety of songbirds. Here, we show that pups of the sac-winged bat (Saccopteryx bilineata), a species with an unusually large vocal repertoire, produce renditions of all known adult vocalization types during bouts of vocalizations, which appear to be independent of a distinct social context. Babbling occurs in pups of both sexes, even though only adult males, not females, utter all different vocalization types produced in infancy. To our knowledge, this is the first evidence of babbling in a nonprimate mammal and suggests that infant babbling may be necessary for the ontogeny of complex vocal repertoires.