Study of 179 specimens of Molossus molossus in Ceará, Brazil, showed morphological and functional ovarian asymmetry in adult females, with the right side more developed. Blastocyst implantation occurred also in the right uterine horn. Active male testicles were bigger (x = 5,38mm) than inactive ones (x = 3,25mm). Pregnancy with subsequent offspring was found in March-April and November, during the wet season.