Pregnancy has been studied in Carollia perspicillata bred in captivity or collected from a reproductively-synchronized wild population on Trinidad, West Indies. In both situations, periods of postimplantational embryonic diapause were sometimes observed. In captivity, this was induced by stress, or once-weekly periods of food deprivation, and resulted in gestation periods of highly variable duration (105-237 days). The normal gestation period for this species is 113-120 days. In the wild, Carollia exhibits two synchronized pregnancy periods. One includes a diapause of at least 44-50 days, but probably somewhat longer, while the other is of normal length. The diapause occurs at the primitive streak stage of development. The postimplantational timing of the diapause is associated with multiple other reproductive specializations, all of which seem to be intended to facilitate blastocyst attachment very soon after entry into the uterus, within a preferred implantation zone. Blastocysts of this species were never observed to have passed that zone or to have a prolonged, free-floating period in the main uterine cavity. This facilitates placental development within an optimally vascularized part of the uterus and the eventual production of a large, highly precocious infant. Comparative immunocytochemical and ultrastructural studies of normal versus delayed pregnancies suggest that inadequate trophoblastic differentiation within the developing placenta may play an important role in maintaining the diapause. In part this may be responsible for delayed invasion of the embryonic side of the placenta by vascularized allantoic mesenchyme. The diapause is also associated with greatly increased trophoblastic invasiveness of unknown significance.