To understand the evolution of local adaptation, the interplay between natural selection and gene flow should be considered. Rhagoletis cerasi L. (Diptera: Tephritidae) is a patchily distributed, stenophagous species of the temperate zone, and the geographical structure of its populations reveals substantial variation in gene flow rates across its distribution range. We studied the demographic components of R. cerasi adults from Greek and German populations by estimating the variability in fitness traits among allopatric populations, as well as among geographically discrete populations, with gene flow. Assuming that body size may exert a profound effect on adult fitness, both thorax and head sizes were considered as covariates in our analyses. Our data demonstrated that females were larger than males in all populations, and adult size varied significantly among populations within groups. Significant differences in a suite of life-history traits of R. cerasi adults were detected among populations with gene flow, whereas there were no consistent differences among allopatric populations. Therefore, the genetic differences among R. cerasi populations, driven mainly by geographical isolation, are poor predictors of variation in the life-history traits of adults. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, ••, ••–••.