The LC50 of a standard preparation of B. thuringiensis Berliner var. kurstaki against 1-day-old larvae of Trichoplusiani (Hübner) was 90 ng/cm2 of diet surface. One-day-old larvae of Anticarsia gemmatalis Hübner and Plathypena scabra (F.) were > 10 × more susceptible than T. ni; Pseudoplusia includens (Walker) was equally susceptible, and Spodoptera exigua (Hübner) and Heliothis zea (Boddie) were ca. 3 × less susceptible. The LC50 against 4-day-old T. ni was 1800 ng/cm2 of diet surface, and the relative ranking for susceptibility of 4-day-old larvae of the other species was similar to that for 1-day-old larvae. Also ranking for susceptibility based on weight of larvae that survived was identical to that based on larval mortality.