There is an inverse correlation between sodium concentration ([Na]) and larval densities of insects in water-filled treeholes, suggesting that sodium might be a limiting resource or that it is controlled by biota. The effects of water [Na] and population density on the growth of the culicidAedes triseriatusand the scirtid beetlesHelodes pulchellaandPrionocyphon discoideuswere studied in a laboratory experiment. At low densities, microcosms high in [Na] produced significantly larger adultA. triseriatusthan those low in [Na]. Scirtids showed the opposite trend. Sodium level had no impact on size at high density for either mosquitoes or scirtids. Dry masses of adult mosquitoes and larval scirtids were significantly smaller at high than at low density. High densities were also associated with longer development and lower survival of mosquitoes. Despite the fact that mosquitoes at low density grew faster at high [Na], the negative effects of high density were not alleviated by high [Na]. High densities of scirtids decreased the [Na] of the water in the microcosms, indicating that the insects may regulate water [Na].