Abstract.Studies on larval population densities and adult emergence rates of the Brugian filariasis vectorsMansonia bonneaeEdwards andMa.divesSchiner were conducted in freshwater swamp forest bordering the Sadong River, Serian District, Sarawak, East Malaysia, during 1984–85.Three species of aquatic host‐plants in the Family Araceae were identified as supporting immature stages of theMa. bonneae/divescomplex. Proportions of positive plants were 4.7%, 6.5% and 3.4% with 6.4±2.6, 7.3±2.8 and 10.1±1.1 larvae per positive plant, respectively, for the plant speciesHomalomena cordataSchott,H.rostrataGriffiths andHydrostemma motleyi(Hook. f.) Mabberley. These data indicate no significant preferences between the three types of host‐plant.Detailed monitoring of the host‐plantH.cordatarevealed no significant monthly fluctuations in larval density per plant nor the proportion of positive plants. 11.6% of larvae wereMa.divesand 88.4% wereMa.bonneae.Mean daily yields ofMa.bonneae/divesadults per square metre ofH.cordatavegetated water surface were 0.45 males plus 0.57 females during the wet season (December‐February) compared with 0.2 males plus 0.31 females during the dry season (June‐August). Thus output of adults per plant was approximately halved, and suitable breeding areas were further reduced, during the dry season. By extrapolation from these rates, a crude mean estimate for productivity ofMa.bonneae/divesfemales is 1.6 million per hectare per annum in swamp forest habitats vegetated with any of the host