A new juvenile hormone mimic [fenoxycarb or RO13-5223 ethyl-(p-phenoxyphenoxy) ethylcarbamate] was evaluated in the field against Culex tarsalis in stagnant water ponds, Psorophora columbiae and Aedes melanimon in irrigated pastures. This material, showing high level of activity against Cx. quinquefasciatus in laboratory, was found to be highly effective against field populations of the 3 former species. A 5% attaclay granular formation yielded 100% inhibition of emergence in Cx. tarsalis at the rate of 0.1 lb/acre ai, 2 days after treatment. The extent of control declined markedly 7 days after treatment. An EC and 1% sand coated granular formulation yielded poor control providing 91 and 69% inhibition of emergence 2 days posttreatment at the high rate of 0.25 lb/acre ai. The 2 flood water mosquitoes were about 10 X more susceptible than Cx. tarsalis. The EC and 1% attaclay granular formulations produced 100% inhibition of emergence of the floodwater mosquitoes Ps. columbiae and Ae. melanimon in irrigated pastures at the rate of 0.01 lb/acre ai, while Cx. tarsalis required 0.1 lb/acre ai for this level of EI. Most of the mortality in treatments with this insect growth regulator was noted in the pupal stage, very few dying in the larval stage or as adults upon emergence. From extensive field studies, it can be concluded that fenoxycarb has excellent potential in operational mosquito control programs.