Eighteen of 47 wastewater evaporation-percolation ponds in Indian River County, Florida were found to contain mosquitoes. Fourteen mosquito species were identified; Culex quinquefasciatus and Cx. Nigripalpus were the most abundant. Two holding ponds receiving secondarily treated effluent from activated sludge wastewater treatment plants were sampled weekly for 17 months to determine seasonal mosquito population trends. At both locations, Cx. nigripalpus dominated the sampling roughly from mid-June to mid-November and Cx. quinquefasciatus was dominant the remainder of the year. Aedes and Psorophora species were found in the 2 ponds in noticeable numbers only in 2 brief periods after the ponds drained and were reflooded. Culex densities are shown to be comparable to another study where nutrient loads were considerably higher.