Females of the European cherry fruit fly,Rhagoletis cerasiL., deposit a pheromone (or a complex of pheromones) immediately after oviposition by dragging their extended ovpositor over the surface of cherries or artificial oviposition devices. The function of this pheromone is the prevention of repeated oviposition in already infested oviposition sites, thus assuring a uniform egg dispersion among available fruits. Apparently it also plays a role in sexual behavior by functioning as a male arrestant. The pheromone is soluble in water and methanol, persisting at least 12 days on the fruit surface under laboratory conditions. Chemotactile but no long-range olfactory mechanisms appear to be involved in its perception by the flies. A simple bioassay and a method to collect pheromone solutions by using artificial oviposition devices is described.