Abstract Synergistic arthropodicidal compositions containing (1) a phosphorus-containing compound of the formula <IMAGE> in which R1, R2 and R3 each independently is an optionally substituted alkyl, alkenyl or aryl radical, at least one of R1, R2 and R3 being an optionally substituted alkenyl radical unsaturated in the 2-position, X is oxygen or sulphur, and Y and Z each independently is oxygen, sulphur or a direct bond; and if both Y and Z are oxygen or sulphur or a direct bond, then any of R1, R2 and R3 can denote propargyl in addition to the radicals indicated above, and (2) at least one (A) carbamate, (B) carboxylic acid ester, (C) phosphoric acid ester other than (1), (D) halogenocycloalkane or (E) halogenoalkane.