Large-scale tests showed that the infiltration of mosquito adults into plots 24 to 30 sq. mi. in area is sufficiently rapid that the plots must be re-treated about every 7-10 days during the peak of the mosquito season. When only 4 sq. mi. were treated, infiltration of annoying numbers of mosquitoes occurred within 48 hrs. Fair protection of the large area was obtained with 4 sprayings during the season. The mean initial reduction with dosages of 0.4, 0.1, 0.05, and 0.025 lb. of DDT per acre were, respectively, 92, 83, 80, and 39%. Except for one test, under unfavorable wind conditions, the avg. reduction for the 0.1 lb. dosage was 92% instead of 83%. The dosage of 0.1 lb. of DDT is considered adequate if applied under suitable wind conditions.