Strategies for the control of Aedes aegypti during urban outbreaks of dengue or yellow fever assume that this species has a maximum flight range of 50-100 meters. Because Ae. aegypti distributes its eggs among several oviposition sites, we postulated that dispersal is driven by the search for oviposition sites, so an ovipositing female may have to fly much further than 50-100 meters to lay all of her eggs. We developed a method for marking Ae. aegypti eggs with a rare alkali metal (rubidium) and showed that in an urban area, oviposition activity in a single gonotrophic cycle lasts several days and covers an area at least 840 meters in diameter (55.4 hectares). We suggest that current practice for the control of dengue and yellow fever transmission by focal treatments with insecticides 50-100 meters around presumed or confirmed cases is unlikely to be effective. Moreover, source reduction (the elimination of breeding sites) may enhance dissemination of virus-infected mosquitoes by reducing the number of available oviposition sites.