Bacillus thuringiensisBerliner val.kurstakiis used extensively in aerial sprays for control of forest defoliators such as gypsy moth,Lymantria dispar(L.), and western spruce budworm,Choristoneura occidentalisFreeman. Although the short half–life ofB. thuringiensisin the field is believed to minimize its impact on nontarget Lepidoptera, there is increasing concern over the immediate and long-term effects of the widespread aerial application of this microbial insecticide. We examined the toxicity and persistence ofB. thuringiensistoward larvae of tree-feeding swallowtail butterflies (papilio glaucus L.andp. canadensisRothschild&Jordan) and the promethea moth,Callosamia promethea(Drury), on seven of their natural host plants.B. thuringiensisapplied to trees at a rate of 40 BIU/ha with a backpack sprayer equipped with a rotary atomizer nozzle (Micronair) was toxic to early and later (fourth) instars, regardless of the host the larvae were feeding on. Long–term persistence ofB. thuringiensison potted tulip trees placed in exposed or below–canopy locations was monitored at 0, 10, 20, 30, and 40 d after the spray. Toxicity toward early–instarp. glaucuspersisted for up to 30 d in 1992 and 1993 field studies. Survival on trees sprayed withB. thuringiensistended to be lower in below–canopy locations, but differences were not always significant. The results of these studies indicate thatB. thuringiensissprays are toxic to some nontarget lepidopterans for at least 30 d after the spray.