The Mexican free-tailed bat (Tadarida brasiliensis mexicana) is primarily a nonhibernating, migratory, spring-breeding subspecies. Our study considers the anatomical structure, function, and seasonal cyclicity of testes and accessory sex glands. Unlike temperate, hibernating, sperm-storing vespertilionid bats, Tadarida does not store sperm, and its spermatozoa differ from spermatozoa of species with prolonged sperm storage. Recrudescence of the male reproductive tract occurs in late winter and early spring. Mating behavior of males and function of secondary sex glands are in synchrony with an activation of the male reproductive tract (spermatogenesis). These events, which take place in the spring, during and immediately following northward migration, lead to insemination of estrogenic, receptive females.