In plants, phenyalanine ammonia-lyase (PAL) has been related to defense mechanisms against insect feeding and wounding, and its role was investigated in poplar (Populus simonii × P. nigra) regarding induction by third-instar Lymantria dispar larvae feeding and punching mechanical wounding. The activity and mRNA expression level of PAL was evaluated during time courses in relation to L. dispar feeding and mechanical wounding. The constitutive levels of PAL activity in healthy poplar showed no significant differences during 72 h. L. dispar feeding and mechanical wounding caused different responses of protein and transcript levels of PAL. PAL was obviously induced by L. dispar feeding during a 24-72 h period. For mechanical wounding, activity and mRNA expression level of PAL were not significant induced but were mostly inhibited. The results suggested PAL gene may be significantly related to poplar defense against L. dispar attack.