In studies on Dacus dorsalis, D. cucurbitae. and Ceratitis capitata. temperatures and relative humidities recorded from 18 different sites in the U. S. were simulated in large chambers, and the responses of the insects noted. Simulations involved the hourly variations of the natural diurnal cycles through the entire annual range of conditions. The climate sites included 9 localities in California, 3 in Florida, 2 in Texas, and 1 each in Arizona, Indiana, Louisiana, and S. Carolina. When provided with adequate food supplies where required, duration of development of immature stages is almost completely governed by temperatures. Summer conditions of many mainland climates permitted growth rates approaching those found in Honolulu. However, summer temperatures in semi-arid localities such as Fresno and El Centro, California, and Tempe, Arizona, are too hot for continuous fruit fly reproduction. Winter conditions in most climates studied, excepting those of the coastal regions of the Southeast, were too cold causing complete cessation of reproduction and development, as well as high mortalities. Results indicate that optimal climatic conditions for these species exist in c. and s. Florida, s. Louisiana, and s. Texas; that marginal conditions, temperaturewise, exist in a narrow section lying just to the north of these optimal areas; that parts of s. California appear to have marginal to optimal winters, but that the extreme summer temperatures there might interfere with reproduction and subsequent adult build-up. These conclusions may be modified after the study is completed.