Synergism of mixtures of pyrethroids with organophosphorus (OP) compounds in insects is reviewed, and the toxicity of such combinations against Spodoptera littoralis (Boisd.) larvae is reported. Mixtures of one of the pyrethroids cypermethrin, fenvalerate or deltamethrin with one of the OP compounds monocrotophos, profenofos, azinphos-methyl or acephate were assayed at different ratios as 24-h-old dipping residues on alfalfa, which was fed to S. littoralis larvae for 48 h. With most of the binary mixtures containing various OP concentrations in excess of those of the pyrethroids, synergism was demonstrated. In the pairs fenvalerate - azinphos-methyl, deltamethrin - azinphos-methyl and deltamethrin - profenofos, however, no synergism was found. In a detailed investigation with pyrethroid concentrations causing 20% mortality and OP concentrations giving a kill of no higher than ;10%, the above findings on synergism were amply confirmed. A cypermethrinmonocrotophos mixture showed synergism also on cotton leaves sprayed in the field. Synergism could not be demonstrated by topical application of pyrethroid - OP mixtures. © 1986 Springer Science + Business Media B.V.