The flight response of Cotesia kariyai Watanabe (Hymenoptera: Braconidae), a parasitoid of the polyphagous herbivore, Mythimna separata Walker (Lepidoptera: Noctuidae), to pairs of different plant species infested by M. separata larvae was tested under a dual choice condition in the laboratory. The oviposition-inexperienced (naive) wasps showed preference in the order: corn > kidney bean > Japanese radish. Wasps that had previously oviposited on the less preferred plant in a pair were found to have shifted their preference to this plant at 2 h after oviposition. However, this shift became indistinct at 17 h after oviposition. Prior oviposition on a plant species other than those being compared also affected the preference. These data suggest that learning is involved in the wasp's flight response. Prior oviposition was also observed to have an effect on the antennal searching behavior of the wasp on corn leaves. Such behavioral plasticity may enhance the efficiency by which C. kariyai searches for polyphagous hosts.