Summary. The major reproductive events in the male eastern pipistrelle, are similar to those of other hibernating vespertilionids. The eastern pipistrelle stores epididymal spermatozoa throughout hibernation, a time when the testes are involuted but accessory gland activity is maintained. However, this species differs from others in that epididymal and testicular spermatozoa persist longer and the weights of the accessory glands are not strongly differentiated between winter and spring/summer. It is suggested that the reproductive period is extended in this species as a function of a more prolonged period of hibernation, resulting in only a brief period of sexual quiescence in mid-summer. The eastern pipistrelle (Pipistrellus subflavus) resembles the canyon bat (P. hesperus) in that some testicular spermatozoa persist during winter. Many aspects of the reproductive anatomy and chronology of these two species are similar; however, eastern pipistrelles apparently lack a seminal vesicle and possess a distinctly different baculum.