This small free-tailed species has been reported from Nicaragua by Felten (1957:14), who listed two females from Corinto. Our records indicate that it is widely distributed, but of localized occurrence. Specimens from Potosí and Santa Rosa were captured in mist nets over streams (as described in the accounts of _Noctilio leporinus_ and _Myotis elegans_, respectively). The specimen from Hda. San Isidro was shot in flight, whereas those from Rivas were captured in a daytime retreat in a deep crevice in a concrete school building. We have no precise knowledge of the conditions under which bats from 3 mi SW Managua were obtained but suspect they were taken from a building. Females in our series were reproductively active at all times for which we have information--early March through mid-July--as follows: a female from Potosí (6 March) carried an embryo that measured 5 mm (crown-rump length) as did one from Santa Rosa (21 March, 17 mm in length); two of three females taken 3 mi SW Managua on 28 March were pregnant (embryos 13 and 15 mm), whereas each of two collected there on 3 May were gravid (embryos 27 and 30 mm); one female from Rivas (25 June) carried an embryo (30 mm) and another was lactating and accompanied by a small (forearm, 19.7 mm) hairless juvenile; and, finally, one of six females netted at Santa Rosa on 13 July was pregnant (embryo 23 mm), but the others evinced no reproductive activity. An adult male, taken at Rivas with the females mentioned above, had testes that were 6 mm in length. Bats of the genus _Molossus_ are in need of thorough systematic study. We think most, if not all, mainland populations of small _Molossus_ with pale-based hairs pertain to the species _Molossus molossus_, originally described from the Lesser Antilles. Although there is some variation in size and overall color among our Nicaraguan samples, all seem to represent that species. Because specimens from Nicaragua closely resemble examples of _M. m. aztecus_ from Jalisco, México (Table 3), in external and cranial proportions, we tentatively refer them to _aztecus_. In our field experience in both Middle and South America, colonies of _M. molossus_ tend to be extremely localized, with individual populations or demes frequently separated by many miles of territory in which the species seemingly does not regularly occur. TABLE 3.--Selected measurements of adult _Molossus molossus_. Table Legend: Col. A: Number of specimens averaged or catalogue number, and sex Col. B: Length of forearm Col. C: Greatest length of skull Col. D: Condylobasal length Col. E: Zygomatic breadth Col. F: Mastoid breadth Col. G: Breadth of braincase Col. H: Postorbital constriction Col. I: Length of maxillary toothrow Col. J: Breadth across upper molars