Abstract: Harpin_Ea secreted by Erwinia amylovora elicites anaphylaxis in tobacco leaves.The seedlings of different tobacco varieties were injected with Harpin_Ea solution at different concentration.It was found that the highest phenylalanine ammonialyase(PAL) activity was induced by 30 μg/mL Harpin_Ea.The study of the relationship between PAL activity and sampling site indicated that the leaf immediately above the injected leaf had higher PAL activity than the opposite half leaf and the leaf immediately under the injected one.The analysis of relationship between PAL activity and the resistance of different tobacco varieties to root-knot nematodes and TMV indicated that all elicited resistant varieties had the same PAL activity profiles versus time;the PAL activities in resistant and susceptible varieties differed obviously,and the PAL activity in resistant varieties was significantly higher than that in susceptible varieties 4 days after eliciting;PAL activity positively related with the resistance of tobacco.