When oriental fruit flies,Dacus dorsalisHendel, of mixed sex were supplied with sugar treated with 0.026% thiotepa for 3 days after eclosion no eggs hatched. Only 0.2% hatched when the dose was 0.0065% thiotepa. A dose of 0.10% was toxic to the insects. Mediterranean fruit flies,Ceratitis capitata(Wiedemann), were not completely sterilized at any dose tested although eggs hatched when flies of mixed sex were supplied with 0.051% thiotepa, and only 0.4, 0.7, 0.7, and 0.8% hatched when the dose was 0.051, 0.026, 0.013 or 0.0065%, respectively. A dose of 0.82% was toxic. Mortality of both species was lower among treated flies that were fed normally after treatment, especially among those treated at or near the threshold dosages, than among untreated flies. Also, when both species were deprived of food and water, treated flies survived longer than untreated flies. Treated oriental fruit fly males were 84% as competitive in mating with females as their untreated counterparts; treated male Mediterranean fruit flies were 79% as competitive.