Pregnancy was studied in short-tailed fruit bats, Carollia perspicillata, both maintained in a captive breeding colony and collected from a reproductively synchronized wild population on the island of Trinidad. Gestation periods for captive females that successfully reared their young varied as follows: mated at a regular oestrus during their first year in captivity (105–178 days) (mean ± sd: 145 ± 19 days); mated at a postpartum oestrus during their first year in captivity (110–158 days) (133 ± 16 days); mated during their second year in captivity (113–169 days) (127 ± 12 days); females born and mated in captivity (113–159 days) (119 ± 9 days). Most females in the last group had gestation periods of 113–119 days; this may represent the normal (nondelayed) gestation period for the species. Histological studies established that most of the observed variation in duration of gestation was due to delays occurring after the completion of implantation. It seems likely that stress, rather than age, was responsible for the prolongation of pregnancy in some animals, because this occurred less frequently in both younger and older females. There may be stressful situations in the wild (for example, lack of sufficient food or roosting sites) in which the ability to delay pregnancies would be of considerable adaptive value. Evidence was obtained that under some circumstances Carollia can extend gestation even further. Many wild-caught females successfully gave birth at 160–229 days after being isolated from breeding males in captivity. These had been captured at the time of year when, based upon subsequent histological studies of field collected specimens, most adult females should have been in early pregnancy. The field studies have also provided evidence that females in the wild population exhibit a seasonal prolongation of pregnancy.