HybridPopulusplants (clone NC 5339) genetically engineered with aBacillus thuringiensis cry1A(a)d-endotoxin gene were planted in the field in the summer of 1993 and evaluated for insect resistance 3 times during the 1994 growing season. Foliage of plants containing the gene encoding thed-endotoxin elicited reduced feeding in bioassays conducted during June, July, and August. Larvae of forest tent caterpillar,Malacosoma disstriaHübner, and gypsy moth,Lymantria dispar(L.), feeding on transgenic leaves consumed significantly less foliage and experienced reduced wet-weight gains compared with larvae fed control foliage. Mortality of early 3rd-instar forest tent caterpillar and gypsy moth feeding on transgenic foliage was greater than for larvae feeding on foliage of controls. Mortality of late 3rd-instar gypsy moth feeding on transgenic foliage did not differ from control larvae.