Myotis nigricans is the most widely distributed and abundant species of the genus in South America. This species occurs in virtually every tropical and subtropical forest association on the vegetation map, as well as in areas of savanna and scrub (Wilson and LaVal 1974). Low, middle, and higher elevation forests, gardens, agricultural areas; nocturnal; flies in more open areas, along trails, streams, etc.; roosts in trees, rock crevices, buildings. Insectivorous feeding habits; mostly small flies, mosquitoes, but also small moths. In Panama, fertilization and implantation occur in late December and early January; first parturition peak happens in February. A second and third birth peaks appear in April-May and August, respectively (Wilson and LaVal 1974). It maintains large colonies, from hundred to thousands; presence of both sexes and juveniles is usual, but males are always on small proportions, suggesting some social hierarchy within the group. The colony becomes usually active for foraging during the first hour after sunset, remaining active all night and returning to the roost just one hour before sunrise (Wilson and LaVal 1974).

This species is found from Nayarit and Tamaulipas (Mexico) to Peru, Bolivia, northern Argentina, Paraguay, and Brazil; Trinidad and Tobago; St. Martin, Montserrat, Grenada (Lesser Antilles) (Bogan 1978; Larsen et al. 2012; Moratelli et al. 2011, 2013; Wilson 2008). Variation in morphological dimensions as well as molecular divergence found in museum specimens assigned to M. nigricans suggests that the species continues to be a composite (Larsen et al. 2012, Moratelli et al. 2013).

It needs integrative taxonomic revision; morphological and molecular analyses indicate it is still a composite. Occurs in several protected areas. The northern population in Mexico needs protection - subspecies (J. Arroyo-Cabrales, pers. comm.).