Continuous high-intensity (24:0 [L:D] photoperiod) light-induced susceptibility in PI 227687 soybean plants previously resistant to cabbage looper,Trichoplusia ni(Hübner), was evidenced by insect feeding assays and the biology of the insect on diet supplemented with leaf powder from such plants. Ethyl acetate and hexane extractables from leaves of these susceptible PI 227687 plants did not show antifeedant activity againstT. nilarvae. However, when such plants were subsequently placed under a 16:8 photoperiod for 2 weeks they regained their resistance against the pest, as shown by leaf-disk feeding assays and presence of antifeedant activity in the ethyl acetate and hexane fractions. Thinlayer chromatography demonstrated that only one of the resolved components in ethyl acetate and hexane extractables is present in resistant PI 227687 but is missing in susceptible PI 227687, ‘Clark’, and ‘Davis’.