The proposed mosquito control agent ISA-20E was applied to the water surface of field test plots near Bakersfield, California to determine whether or not mortality occurs in populations of indigenous mosquitoes and aquatic nontarget arthropods. Five sampling and monitoring methods were used to determine effects of application rates at 0.25, 0.5 and 1.0 ml/m2 surface area. ISA-20E was effective in reducing 3rd and 4th instars of Aedes nigromaculis and Ae. melanimon populations at 0.5 and 1.0 ml/m2 surface rates, averaging 88% and 96% mortality reduction respectively. It was also effective in reducing pupae of the above species at all 3 rates. For the test on mixed Culex tarsalis and Culex quinquefasciatus populations, mortalities ranged from 49% to 100% for the 3-day test period at the 3 rates used. Nontarget arthropods which showed acute lethal effects were Corixids (Corisella spp.), Notonectids (Notonecta unifasciata), clam shrimp (Eulimnadia sp.), and Tropisternus lateralis beetle adults. Nontargets that did not exhibit mortality were mayfly (Callibaetis spp.) naiads, Chironomid larvae and copepods.