TransgenicPopulusplants (clone NC5339) expressing aBacillus thuringiensisd-endotoxin gene significantly reduced feeding and weight gain by gypsy moth,Lymantria dispar(L.), and the forest tent caterpillar,Malacosoma disstriaHubner. Forest tent caterpillar larvae also experienced significant mortality following exposure to the transgenicd-endotoxin. Gypsy moths were better able to recover after transfer to artificial diet than were forest tent caterpillars. Transgenic leaves deterred feeding by both species in behavioral choice tests. Four additional transformants contained, but did not express thed-endotoxin gene. Results are discussed in terms of efficacy, plant deployment strategies, ecosystem stability, and tree improvement.