For six years we examined the reproductive biology of Common Moorhens (Gallinula chloropus) nesting in an impounded cattail (Typha spp.) marsh in South Carolina, USA. The egg-laying period averaged 78 days. Most (52%) of 82 nests were started in May. The moorhens appeared to be single-brooded. Mean clutch size was 6.55, the same as that reported from field studies in Britain, but significantly smaller than that calculated from South Carolina oology data. Common Moorhens occasionally built their nests on those of Boat-tailed Grackles (Quiscalus major), and in one instance a moorhen laid an egg in a grackle nest. About 18% of moorhen nests were parasitized by other moorhens, which usually added one or two eggs. Over four years, we estimated that 68% of 54 moorhen nests produced at least one fledgling, an estimate close to that reported by researchers in Britain. Most nest mortality probably was caused by snakes, because large mammals and avian predators rarely were noted in the study area. The relatively high reproductive success of Common Moorhens appears to be related to the species' use of extensive stands of cattails growing in a sheltered area with controlled water levels. The moorhens also may benefit from the presence of American alligators (Alligator mississippiensis), which discourage large mammals, and from Boat-tailed Grackles, which mob avian predators.