Bitter gourd, Momordica charantia, was less palatable to 2 species of armyworms, Spodoptera litura and Pseudaletia separata, than 2 other cucurbitaceous plants. A methanol extract of M. charantia leaves inhibited feeding of the armyworm larvae. The 2 most active fractions obtained by silicagel chromatography were purified by HPLC. Momordicine II, triterpene mono-glucoside, was identified as an antifeedant compound from the most active of these fractions. The second active fraction led to the isolation of a new triterpene di-glucoside. Fresh leaves of M. charantia contained ca. 0.3% of momordicine II. Momordicine II showed a significant antifeedant effect on P. separata at the concentrations of 0.02, 0.1 and 0.5% in artificial diets. Momordicine II caused a significant feeding reduction in S. litura only at the highest concentration (0.5%) tested. The difference in the feeding response of the 2 armyworms to momordicine II may be related to the diversity in their host range. The author also examined whether stress applied to plant exerted an effect on insect′s feeding preference. M. charantia is the host plant for the larvae of the pyralid moth, Diaphania indica but not for those of P. separata. Feeding response to UV-irradiated M. charantia leaves was compared between these 2 insects. D. indica preferred intact leaves, while P. separata preferred UV-irradiated leaves. These differences might be caused by the difference in the contents of antifeedants and feeding stimulants in the intact and UV-irradiated leaves.