In Indian River County, Florida, mosquito populations were monitored weekly for 12 months in early and more advanced treatment stages of secondarily treated domestic wastewater and also in untreated primary and secondary citrus packing house washwater. Well defined differences in mosquito species abundances for early vs. late wastewater stages existed. In the early levels of secondarily treated effluent and in the primary citrus washwater, Culex quinquefasciatus populations were dominant. In the later wastewater stages of both systems, Cx. nigripalpus, Anopheles spp. and Uranotaenia spp. were more common. Seasonal Culex spp. abundance patterns were apparent at some but not all study sites. Water chemistry measurements demonstrated that both systems contained relatively low levels of nutrient-related parameters with large concentration variability.