SUMMARYA survey of microfilaraemia among the population of Vanua Levu, Taveuni and Koro islands in northern Fiji was conducted in 1968 and 1969 as a prelude to a campaign of mass treatment with diethylcarbamazine.The prevalences of microfilaraemia were found in the more moist conditions of Taveuni and Koro and on the windward southern side of Vanua Levu to be higher than on the drier northern side of Vanua Levu. On both sides of Vanua Levu prevalences were lower inland than near the coast.Under apparently similar environmental conditions those of Fijian ethnic origin exhibited a higher prevalence of microfilaraemia than that shown by Indians. This ethnic difference and a difference between the prevalences in male and female Fijians are considered to be due more to higher rates of recovery from microfilaraemia in Indians and Fijian women than to diminished exposure to mosquitoes. Mathematical models have been used as an aid to the interpretation of the data, and, where appropriate, comparison has been made with the prevalence of antibodies to dengue, an arbovirus having the same vectors.Household infections were analysed by computer techniques. Infections in large households were not proportionately higher than in small households, indicating that transmission was not intrafamilial. The clustering of infections within households, though present, was not marked. Among the occupants of outlying settlements the prevalence of microfilaraemia was relatively low indicating a lower risk of infection due to isolation.