Residues of Hercules AC-5727 (m-isopropylphenyl methyl carbonate were highly effective against adults of Anopheles quadrimaculatus Say, A. albimanus Wied., Aedes taeniorhynchus Wied., and A. acgypti (L.). In laboratory tests with A. quadric maculates on plywood panels, dosages as low as 1 mg. per square foot produced 100% mortality for at least 20 weeks. The compound was better than DDT, malathion, dieldrin, or Bayer 29493 (Baytex®; O, O-dimethyl 0-[4-(methylthio)-m-tolyl] phosphorothoate). Its outstanding characteristic is the rapidity with which it acts, for a 1-minute exposure to deposits of 100 mg. per Square foot consistently produced 100% knockdown within 5 to 10 minutes throughout the first month. Hercules AC-5727, malathion, and DDT did not have long residual toxicity on white-wash or sun-dried brick. In buildings naturally infested with A. quadrimaculatus, Hercules AC-5727 and DDT were still giving excellent protection after 18 weeks. Both were more durable than malathion.