Methyl parathion was applied to Clear Lake at a rate of 3.3 ppb for control ofChaoborus astictopusDyar&Shannon. Three treatments at 20-day intervals had no lasting impact on zooplankton. Recovery to near or above pretreatment numbers was rapid for the organisms examined, including copepod nauplii,Diaptomus franciscanusLilljeborg,Cyclopssp.,Mesocyclopsspp.,Daphniaspp.,Bosmina longirostris(O. F. Muller),Diaphanosoma brachyurum(Lieven) andCeriodaphnia reticulata(Jurine).No residues were detected in lake sediments. Highest residues in water were found 8 and 24 h after the treatments, varying from 0.5–5.4 ppb, and generally declining after each treatment. However, generally larger residues were found with each successive treatment. Residues in bluegill sunfish,Lepomis macrochirusRafinesque, exposed to the treatments in live cars, varied from 11–110 ppb. A significant correlation was found between the methyl parathion levels in fish and water collected simultaneously.