Experiments in Hawaii have shown that infestations of Dacus dorsalis can be reduced 60 - 100% if male flies are destroyed before they can fertilize the females. In a 28-month experiment on Oahu in a 1.5-mile section of a deep gulch with an abundance of wild fruit fly hosts, 45 - 55 feeding stations treated with methyl eugenol and parathion or Pyrolan attracted and killed more than 3 million male flies. Mean infestations in 4 successive guava crops ranged from 1.2 to 4.2 larvae per pound in the treated gulch and from 7.1 to 35 per pound in similar untreated gulches. On the Hamakua coast a 6-square-mile area was protected for 16 months with 165 to 215 cane-fiber bait stations, each treated monthly with methyl eugenol and Pyrolan. The bait stations kept the central portion of the treated area almost devoid of males for most of the period at a monthly cost of 2 cents per acre. At elevations of 700, 1100, 1500, and 1900 feet mean oriental fruit fly infestations were 74, 70, 82, and 60% less, respectively, than at similar elevations in the untreated areas to north and south. Control of 100% was obtained during several monthly periods. Infestations at each elevation in the treated and control areas leveled off rapidly after termination of the experiment. Suppression of the oriental fruit fly by this method accounted for marked increases in Mediterranean fruit flies, Ceratitis capitata. Parasitization of oriental fruit fly larvae by opiine parasites[long dash]approximately 60%[long dash]was not affected by the treatment. A 7-acre experiment showed that the male-annihilation method is not suitable for small areas because of immigration of fertile females. It should be most effective when applied to whole or well-isolated populations, or when used in a combined scouting and control operation to combat incipient oriental fruit fly infestations on the mainland.