In response to herbivore damage, several plant species emit volatiles that attract natural predators of the attacking herbivores(1-5). Using spider mites (Tetranychus urticae) and predatory mites (Phytoseiulus persimilis)(1-4), it has been shown that not only the attacked plant but also neighbouring plants are affected, becoming more attractive to predatory mites(3,6) and less susceptible to spider mites(6). The mechanism involved in such interactions, however, remains elusive. Here we show that uninfested lima bean leaves activate five separate defence genes when exposed to volatiles from conspecific leaves infested with T. urticae, but not when exposed to volatiles from artificially wounded leaves. The expression pattern of these genes is similar to that produced by exposure to jasmonic acid. At least three terpenoids in the volatiles are responsible for this gene activation; they are released in response to herbivory but not artificial wounding. Expression of these genes requires calcium influx and protein phosphorylation/dephosphorylation.