Hybridization can provide a window into how populations diverge to form new species. Here, we confirm hybridization between Rhagoletis completa Cresson, 1929 and Rhagoletis zoqui Bush, 1966, two species of walnut husk-infesting flies that geographically overlap in a narrow area of parapatry in Northeastern Mexico. Rhagoletis completa and R. zoqui are members of a species group (Rhagoletis suavis) that has been hypothesized to speciate in allopatry. Sexual selection has been argued to be a potentially important factor for generating pre-mating isolation among walnut husk flies, because of the differences in wing morphology and body coloration, and the existence of sexual dimorphism within species. However, there was no evidence for pre-mating isolation between R. completa and R. zoqui, based on choice and no-choice mating experiments conducted on adults of fly populations outside the contact zone. There was also no support for reduced fertility of hybrid matings or for F1 inviability; however, F1 hybrids appeared to have lower fertility and F2 offspring have reduced survivorship. Postzygotic isolation in later generation hybrids of mixed ancestry therefore appears to be the first intrinsic barrier to gene flow evolving between R. completa and R. zoqui. We discuss the implications of our results for allopatric speciation in walnut flies and the potential evolutionary fate of R. zoqui and R. completa if they were to come into broad geographic contact in the future. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, ••, ••–••.