In practical tests conducted near Tallahassee, Fla., and Stuttgart, Ark., DDT applied in various carriers at dosages ranging from 65 to 400 mg. per sq. ft. protected buildings from natural infestations of A. quadrimaculatus adults as long as 70 days. Complete elimination was obtained with a large % of the treatments. When mosquitoes were present in the treated buildings, the number was always much fewer than prior to spraying or in untreated buildings used as checks. The chemical activated mosquitoes and caused many of them to leave the building, but a large % of those that escaped had obtained a lethal dose. In one test numerous Psorophora spp. adults were killed when a light was used to attract them to a treated building. In a simulated practical test 100% mortality was obtained over an 84-day period when laboratory-reared A. quadrimaculatus adults were confined in a treated room for 24 hrs. On one occasion, however, some mosquitoes continued to bite a host exposed in this room until a knockdown of 99% had occurred.