Abstract: The host selection of Diadegma semiclausum between plants of two Brassica species and the effect of plant volatiles on the host-selection behavior of D. semiclausum were studied in the lab. When D. semiclausum wasps were provided with equal numbers of larvae of Plutella xylostella on Chinese cabbage (Brassica campestris ssp. pekinensis) and common cabbage (Brassica oleracea var. capitata), the number of hosts parasitized on Chinese cabbage was higher than that on common cabbage. In bioassays with volatiles, D. semiclausum wasps showed preference for the Chinese cabbage with P. xylostella larvae over the common cabbage with P. xylostella larvae, the wasps also showed preference for Chinese cabbage plants previously infested by P. xylostella larvae over the common cabbage plants previously infested by P. xylostella larvae. However, the wasps showed no preference between the two plant species when the plants were intact or artificially damaged.