Larvae of Dacus dorsalis (Hendel), the oriental fruit fly, were exposed to cw ultrasound at frequencies of 43, 123 and 447 kHz, with eggs also exposed to 43 and 447 kHz. Mortality as a function of ultrasonic intensity and duration of exposure was determined. At 43 kHz, no eggs or larvae were found to survive 0.5 W/cm2 or 0.2 W/cm2, respectively. Also at higher frequencies the larvae were more sensitive to ultrasound than were the eggs. A dramatic increase of lethal intensity with frequency suggests cavitation as the mechanism by which mortality is induced.