Because agricultural insecticides have potential to disrupt biological control of mosquitoes, wequantified whether an insecticide used in rice fields causes mortality of mosquitofish. Laboratory studies haveshown that lambda-cyhalothrin (Warrior™) is toxic to fish; however, some studies report low field toxicities ofpyrethroids to fish because they degrade rapidly and adsorb to soil. We tested whether Warrior kills mosquitofishunder field conditions. Replicated enclosures in a rice field were either sprayed with Warrior at 5.8 g activeingredient/ha or were untreated. Mosquitofish were either added before the spray, or 7 days later. Of those addedbefore the spray, none survived. Most fish added 7 days later survived.