The soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae) is a severe pest of soybean. Soybean aphid populations cycle between two hosts. The primary host complex in North America includes species belonging to the Rhamnus genus and the secondary host is soybean. The potential olfactory cues produced by the interaction of the soybean aphid with its plant hosts may represent novel cues for existing parasitoid species in North America. In this study, Aphidius spp. parasitoids were evaluated with respect to their responsiveness to volatiles originating from the primary host, Rhamnus cathartica L. and/or a host / host-plant complex consisting of the soybean aphid feeding on R. cathartica. Three species were evaluated in two-choice olfactometer bioassays with undamaged R. cathartica (A. ervi, A. matricariae and A. colemani). A. colemani responded to both aphid-damaged and undamaged R. cathartica.