Rhagoletis pomonella is a model for incipient sympatric speciation (divergence without geographic isolation) via host plant shifts. We are testing the hypothesis that R. pomonella flies originating from hawthorn, apple, and flowering dogwood fruit use fruit volatiles to distinguish among their respective host plants. Solid phase microextraction combined with gas chromatography and electroantennogram detection were used to identify unique blends of volatiles from each fruit type. In flight tunnel assays flies preferentially flew upwind (>70%) to the volatile blend from their natal host. A low percentage of flies also flew to non-host blends, and >90% of these flies were ones that flew to their natal blends as well. Thus, there appears to be a percentage of flies in each population that are broadly tuned with respect to fruit odor blend specificity. Field tests also showed that over the fruiting season significantly more flies were captured with natal fruit volatiles. Because R. pomonella rendezvous on or near the unabscised fruit of their hosts to mate, the behavioral preference for natal fruit odor translates directly into premating reproductive isolation between the fly races. However, because some flies are less discriminating there is the potential within each population for host shifts to occur.