Immediately following oviposition (but rarely following attempted boring without oviposition), apple maggot, Rhagoletis pomonella (Walsh), females were observed to circle around the fruit for about 30 seconds, dragging their fully-extended ovipositors on the fruit surface behind them. The results of field and laboratory experiments indicated that during such dragging, a pheromone deterring repeated boring is deposited on the fruit surface. Where the fruit was small (15-mm-diam cherries) and the drive to oviposit moderate (as among the field population), there was almost total deterrence for 4 days (i.e. until termination of the experiment) to any repeated boring in a fruit so marked by a single female. Where the fruit was large (55-mm-diam apples) or the drive to oviposit strong (as among laboratory flies deprived of oviposition sites), marking by only one female was insufficient to elicit strong deterrence. It is suggested that deposition of this marking pheromone is an important factor mediating uniformity in egg dispersion among available fruits, and that the amount of fruit surface area marked following a single oviposition is related to the amount of food or space required by one larva to grow to maturity. The potential utility of the pheromone in population suppression is also discussed.