Monitoring of adult flight patterns of the blueberry maggot fly, Rhagoletis mendax Curran, in New Jersey, indicated that the adults are active over a much longer period than previously reported. Captures on Pherocon AM traps over two seasons in wild sites and commercial fields of highbush blueberry, Vaccinium corymbosum L. (Ericaceae), showed that adult flies are present for most of the period from early July to early November. Trap captures in wild sites peaked during July–August, whereas in some commercial fields, peak captures were recorded in September. Emergence patterns were determined by collecting pupae from a wild site and a commercial field at the time of peak fruit infestation. The following year, the wild-site and commercial-field populations showed distinct emergence periods that were in broad agreement with trap captures at these locations. Comparison of an allozyme locus, using individuals collected in commercial blueberry fields, both on Pherocon AM traps and from infested fruit, confirmed that these populations were R. mendax and not any of the sibling species with a similar flight period. These data show that there are considerable phenological differences between some R. mendax populations. Given this plasticity, current debates on evolutionary mechanisms in flies of the genus Rhagoletis Loew should consider that the flight period of R. mendax is probably neither a major limiting factor in the use of hosts with different fruiting schedules nor an effective premating isolation mechanism with respect to other sibling species.