I studied reproduction and postnatal ontogeny in a captive population of the African water rat, Dasymys incomtus, whose numbers have declined since the 1960s. Mean litter size was 2.9, gestation was 29 days, and maximum reproductive output was 18 young. Neonates were altricial and weighed 6.1–10.3 g. Weaning occurred at 24 days, and sexual maturity was attained at 6 weeks in males and 17 weeks in females. The estrous cycle was 7–16 days, the variation being caused by prolonged estrus. Comparisons with other African murid rodents suggest that the low reproductive output and prolonged postnatal ontogeny of D. incomtus reflect phylogenetic constraints imposed by an altricial strategy and nipple clinging but also may be adaptive responses to the stable and predictable mesic habitats occupied by D. incomtus. However, the low reproductive rate compromises its ability to cope with continued habitat destruction and desiccation, and populations of D. incomtus can be expected to decline further.