Comparisons were made of the toxicity of residual sprays of pyrethrins and DDT to adult A. quadrimaculatus and Aedes aegypti mosquitoes over a 3-month period. The results indicated that applications of pyrethrins at 200 mg. per sq. ft. produced total knockdown in 1/10th to 1/5th the time required for DDT at the same rate of application. However, some revival occurred from exposure to the pyrethrins, while none occurred from the DDT. No marked deterioration in toxicity of the pyrethrins treatment resulted from exposure to indoor light conditions. Residual pyrethrins treatment may be more effective than the similar application of DDT in the prevention of mosquito biting in dwellings,.