The literature is reviewed and the observational and experimental facts weighed with reference to the theory that in bats copulation typically (two exceptions are known) occurs in the fall, the spermatozoa lying in the uterus or the vagina until spring, when they function in fertilization. This theory has been called in question by Cuyler and Hartman; but recent work on the physiology of spermatozoa, notably that of Redenz, render it plausible. Nevertheless the theory has not yet been tested by isolating inseminated females from fall or winter till spring.