The apple maggot, Rhagoletis pomonella (Walsh), occurs commonly in black hawthorn in Utah, attacks tart cherry infrequently, and has generally failed to infest apples. This study investigated how the timing of fruit maturation might account for this pattern. Infestation of hawthorn at three sites in 1988 commenced when fruits were rapidly increasing in size and water content, and perhaps most significantly, when mean resistance to penetration of the fruit skin dropped below a threshold. Dissections of trapped females indicated that a significant fraction of flies possessed mature ovaries before oviposition occurred in hawthorn in the field. The importance of fruit toughness early in the season also was suggested by measurement of individual fruits in 1989; no fruit with a penetration resistance >60 kg/cm2 yielded a fly puparium. No puparia were obtained from three apple cultivars that were sampled from an unsprayed orchard adjacent to a densely infested hawthorn stand. When apples collected during the oviposition period in hawthorn were presented to caged flies, only the early-maturing cultivar was successfully infested. Penetration resistance of mid- and late-season cultivars remained high throughout the period of fly activity. These results suggest that flies that originate in hawthorn in Utah emerge too early to attack the dominant local cultivars of apple. In contrast, the timing of adult emergence is relatively late for infestation of cherry, which ripens well before hawthorn. In behavioral assays, fruit phenology had only a minor effect on fruit acceptance; ovipositing females were less likely to attempt oviposition in immature hawthorn or cherry fruits but failed to discriminate between nearly ripe and ripe fruits.