In Indian River County, Florida a 2-cell salt-marsh mosquito control impoundment flooded since 1976 with secondarily treated wastewater has produced mosquitoes in negligible numbers, although some inland holding ponds produce Culex mosquitoes in great densities. The use impoundments to provide tertiary treatment to nutrient-rich wastewater reduces nitrates but not orthophosphates. Salinity, nitrate, and orthophosphate levels differ significantly from other impoundments not receiving wastewater as is evidenced by vigorous growth of some salt-marsh and freshwater vegetation. This method of wastewater disposal can be economically beneficial to mosquito control districts in helping control mosquitoes while at the same time improving water quality.