Pyriproxyfen (S-31183) was applied to dairy wastewater lagoons, during 1988, at 0.1 kg (AI)/ha in single and multiple applications which resulted in control of Culex spp. larvae for periods of 7 to 68 days. Length of the control period appeared to be related to water quality, with greater residual efficacy in more polluted sources. The AI apparently adsorbed onto organic debris where efficacy remained high in the lagoon even after water was pumped from the lagoon and replenished with untreated wastewater. Alternating treatments with control agents having a different mode of action is suggested to avoid selection of insecticide resistance.