In Indian River County, Florida mosquito control impoundments, larval mosquito sampling and hydrological measurements demonstrated the importance of careful consideration of these factors when developing management plans for impoundments used for wastewater retention. Discharging secondarily treated wastewater into an impoundment resulted in only minor mosquito production. However, a treatment plant failure produced extremely high Culex densities in the impoundment. Average water loss rates in impoundments studied were due to evapotranspiration (0.25 cm/day) and percolation (0.38 cm/day). Greatest percolation (0.68 cm/day) was measured when the impoundments were maximally flooded. Under the conditions of this study, the impoundments can assimilate approximately 124 cm/year of wastewater (1.52 million liters/day) over a 50 ha area without overflows.