A novel A,B-seco-tetranortriterpenoid lactam, named munroniamide (1), along with three known ceramides (2-4), was isolated from the methanolic extract of the whole bodies of Munronia henryi. The structure of 1 was established using spectroscopic methods. Compound 2 exhibited significant antifeeding activity, and 1 exhibited moderate activity, whereas 3 and 4 showed negative activity against Pieris brassicae L.