Sections of the upper or lower surfaces of leaves of cabbage plants were painted with the cabbage looper,Trichoplusia ni(Hübner), single-embedded nuclear polyhedrosis virus (100 PIB/mm2). Discs of leaves sampled between 0 and 28 days after treatment were fed to individually reared first-instarT. nifor 48 h. The larvae were subsequently fed semisynthetic diet and held to determine mortality. The insecticidal activity of the virus placed on the lower leaf surfaces was more persistent (50% mortality of test larvae after 3 weeks), compared with the activity of the virus on the upper leaf surfaces (13% mortality after 5 days). The importance of virus placement on the plants and of the feeding behavior of the target insect on virus effectiveness is discussed.