A laboratory experiment was conduct at the Department of Entomology, N. M. College of Agriculture, N.A.U., Navsari to assess the effect of water quality on efficacy of three organophosphorus pesticides viz. monocrotophos, chlorpyriphos and quinalphos against third instar larvae of Spodoptera litura revealed that water EC and efficacy of pesticide have positive relationship i.e. water EC increases the larval mortality also increased. Higher the EC (16 EC, 12 EC), tube well, canal and distilled waters were found equally effective against larvae of S. litura. While, calcium level of water and efficacy of pesticide have negative relationship i.e. level of calcium increases in water, efficacy of monocrotophos, chlorpyriphos and quinalphos decrease. The tube well, distilled and canal waters along with insecticides had recorded higher mortality as compared to different calcium water treatment.