Bats constitute a substantial proportion of mammal diversity within the Asian tropics and subtropics and are particularly susceptible to population losses associated with human activities. This poses a conservation concern in Asian karst areas which support high bat species diversity, yet are experiencing habitat loss and degradation and increasing pressure from tourism and extractive industries. As disturbance during crucial reproductive periods (late pregnancy, lactation and weaning) threatens reproductive success, we investigated the reproductive phenology of a bat assemblage at two karst sites in North Vietnam. Our results indicate that the timing of major reproductive events coincides among two cave-dwelling pteropodids, and among 26 cave and foliage dwelling rhinolophids, hipposiderids and vespertilionids. March–July is the primary reproductive period for all insectivorous species sampled, and protection of maternity roosts during this time is critical. Reproduction in cave-dwelling pteropodids spanned a greater period (March–December), due to two birth periods each year. Lactation in the three insectivorous families studied was positively correlated with rainfall and temperature, with weaning occurring during the peak wet season. The strong congruence in reproductive phenologies in our results and climatic homogeneity of North Vietnam (18–23°N) suggests that our study may have wider applicability within the region. Vietnamese caves support high bat diversity which is likely threatened by harvesting for consumption and tourism development nationwide. Studies to investigate and address these threats should be given high priority.