Mating in the long-fingered bat (Miniopterus schreibersii) from the East Cape region of South Africa occurred during a four-week period in April and May, and implantation was delayed until August. Analyses of foetal age and estimated dates of conception indicate that after implantation, the progression of pregnancy within the population was synchronous both within and between years. It is suggested that implantation, probably initiated by increasing day length, occurred at about the same time throughout the population and thus synchronized reproduction. In the final month of pregnancy foetal growth rate and, consequently, the timing of parturition in the three years of the study varied. These variations were correlated with variations in rainfall and it is suggested that the failure of the November rains in 1990 would have caused a reduction in insect abundance and that this was responsible for the decrease in foetal growth rate and the delay in parturition.