Larvae of the diamondback moth, Plutella xylostella L. (Lepidoptera: Plutellidae), a crucifer specialist, refuse to feed on the wintercress, Barbarea vulgaris R. Br. (Brassicaceae). The mechanism of this unacceptability to P. xylostella was evaluated by a series of no-choice bioassays using leaf disks. Topical application of sinigrin enhanced feeding of the larvae on the leaf disks of a non-host plant, lettuce (Lactuca sativa L.), but not on those of B. vulgaris. Application of the crude methanolic extract from B. vulgaris leaves deterred P. xylostella feeding on the leaf disks of cabbage (Brassica oleracea L.) in a dose-dependent manner. This feeding deterrent activity was transferred predominantly to the chloroform fraction, but not to the n-hexane or water fractions. These results suggest that the unacceptability of B. vulgaris to P. xylostella larvae is mainly due to chemical constituent(s) extractable with chloroform.