The concept of "host fidelity," where host-specific mating occurs in close proximity to the oviposition site and location of larval development, is thought to impart a pre-mating isolation mechanism for sympatric speciation ( sensu members of the genus Rhagoletis). The apple maggot fly, Rhagoletis pomonella, and the blueberry maggot fly, R. mendax, are morphologically similar sibling species thought to have speciated in sympatry by divergence of host plant association. Both of these fly species are attacked by the specialist braconid parasitoid, Diachasma alloeum. The current study demonstrates that both male and female D. alloeum exhibit a behavioral preference for the odor of the fruit of their larval Rhagoletis host species. Specifically, those D. alloeum emerging from puparia of R. pomonella are preferentially attracted to hawthorn fruit and those emerging from puparia of R. mendax are preferentially attracted to blueberry fruit. However, male D. alloeum reared from either R. pomonella or R. mendax were equally attracted to females originating from both Rhagoletis species. We suggest that the data herein present evidence for " host fidelity," where populations of D. alloeum exhibit a greater tendency to mate and reproduce among the host plants of their preferred Rhagoletis hosts. Furthermore, host fidelity may have resulted in the evolution of distinct host races of D. alloeum tracking the speciation of their larval Rhagoletis prey. Rhagoletis mendax (Tephritidae) : Host, Parasite Rhagoletis pomonella (Tephritidae) : Host, Parasite Diachasma alloeum (Braconidae) : Parasite Evolution; Behaviour; Orientation; Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts; Zoogeography Diachasma alloeum; Speciation; Sympatric speciation; Taxis; Chemotaxis; Dipteran hosts; Chemotactic response to host plant hosts; host fidelity & speciation implications; Rhagoletis mendax & Rhagoletis pomonella; Sympatry Rhagoletis mendax; Rhagoletis pomonella; Hymenopteran parasites; Parasites response to host plant hosts; host fidelity & parasite speciation implications; Diachasma alloeum; Plant hosts none