From the lagoon at Fangataufa Atoll, French Polynesia, forty-two species of harpacticoid copepods belonging to 21,genera were identified; over a third of them are new. The assemblages were largely dominated by epipsammic forms of the family Diosaccidae. Some endopsammic harpacticoids were found in deep water silty-sands, but mesopsammic forms were very rare. The harpacticoid community structure of the lagoon appears to be determined primarily by sediment granulometry as controlled by reef hydrodynamics. The harpacticoid copepod assemblages permit the identification of two major zones within the lagoon, namely the muddy deep central zone, and the inner sandy fringe at the periphery.