Impregnated filter paper, and coated glass substrata, as well as direct spraying techniques were employed to test the relative toxicity of several organic molecules related to DDT, as contact insecticides against adult Drosophila melanogaster. Anaesthetizing the flies for convenience of handling did not affect observed mortalities. In general both impregnated paper and coated glass (vial) techniques gave reproducible results. The former resulted in an unexplained drop in mortalities above a certain dosage level, the latter gave somewhat more uniform results. Direct spraying was superior when the chemical had a fumigant effect also.γ, 1,2,3,4,5,6-Hexachlorocyclohexane and the difluoro analogue of DDT, alone out of 18 chemicals tested, exceeded DDT in effectiveness. The γ isomer of 666 exhibited a strong fumigant but also a long residual effect while the difluoro analogue lost toxicity rapidly.Removal of one or both phenyl chlorine atoms from the DDT molecule, or alteration of their p-p positions, or their substitution by any radical other than F, interference with the trihalogen group of DDT, or the introduction of a double bond into the molecule, all seriously affected the insecticidal action of the DDT. Arranged from the most to the least toxic, the halogen analogues of DDT have the following order: fluorine, chlorine, bromine, iodine.