This research was designed to test the hypotheses that 1) phenolic glycosides can account for differential performance of gypsy moth (Lymantria disparL.) on quaking aspen (Populus tremuloidesMichaux) and otherPopulusspecies, and 2) midgut esterases are involved in detoxication of phenolic glycosides by gypsy moth larvae. We bioassayed tremulacin, a phenolic glycoside, with and without S,S,S,-tributylphosphorotrithioate (DEF), an esterase inhibitor, against first and fourth instars. We also measured activities of β-glucosidase, esterase, and glutathione transferase enzymes from larval midguts. Tremulacin did not alter first instar survival but greatly prolonged duration of the first larval stadium. Tremulacin at the 2% level did not alter performance of fourth instars, but at the 3% level it reduced growth through effects on approximate digestibility and food conversion efficiency. DEF strongly potentiated the effects of tremulacin, suggesting that esterases are important for detoxication of phenolic glycosides by gypsy moth larvae. Enzyme assays revealed moderately high esterase activities, which were not induced by larval consumption of tremulacin. We suggest that high levels of tremulacin and related compounds in somePopulusfoliage may overload the detoxication capacity of gypsy moth larvae, leading to impaired performance.