THE insecticidal properties of valone (2-isovaleryl 1,3-indandione) were being investigated some years ago1, when the material was apparently eclipsed by the discovery of DDT. Recent tests on insect pests of stored products have shown that, although not outstandingly insecticidal by modern standards, valone has an unusual combination of properties; namely, (1) if the dose is high enough to be lethal, it kills the insects rapidly, (2) in sub-lethal doses it produces a paralysis from which insects afterwards recover, and (3) the percentage mortality is unusually sensitive to changes in dose. (2) indicated that insects can detoxify valone. By analogy with the probable mode of action of synergists for pyrethrins2 and DDT3, that is, depression of the normal detoxification of the insecticide, it seemed possible that the insecticidal action of valone could be intensified by a suitable synergist. Furthermore, (3) suggested that the synergist need not be highly efficient in order to increase the mortality considerably.