In laboratory tests DDT has been found effective as a residual-type spray against adults of Anopheles quadrimaculatus and Aedes aegypti, when applied on wooden boxes and canvas cages at the rate of 10-400 mg. per sq. ft. Of the DDT solns. tested, orthodichlorobenzene, ethylene dichloride, and cyclohexanone produced the most toxic residue on wood, while kerosene, benzene, cyclohexanone, ethylene dichloride, and a water emulsion containing DDT, xylene and "Wetsit" were most effective on canvas. Comparative tests showed a water emulsion and a water suspension to be equally effective on wood, and both slightly superior to a kerosene spray. An exposure period of at least 2-4 hrs. was required before all mosquitoes ob- tained a lethal dose. Many of the residues were still completely effective after 32 weeks when 24-hr. exposures were used. Sunlight had a deleterious effect, but the loss in toxicity occurred very slowly, for a deposit of 50 mg. did not fail to kill mosquitoes until after the 24th week. DDT residues were highly toxic on unpainted surfaces and on surfaces covered with 2 coats of cold water-casein paint that had been applied < 2 weeks. The residue was much less effective, however, on surfaces recently painted with oil paint. The chemical activated the mosquitoes that rested on treated surfaces and caused the insects to fly, but not before most of them had obtained a lethal dose.