Plant roots in the soil are under attack from many soil organisms. Although many ecologists are aware of the presence and importance of natural enemies in the soil that protect the plants from herbivores, the existence and nature of tritrophic interactions are poorly understood. So far, attention has focused on how plants protect their above-ground parts against herbivorous arthropods, either directly or indirectly (i.e. by getting help from the herbivore’s enemies). This article is the first in showing that indirect plant defences also operate underground. We show that the roots of a coniferous plant (Thuja occidentalis) release chemicals upon attack by weevil larvae (Otiorhynchus sulcatus) and that these chemicals thereby attract parasitic nematodes (Heterorhabditis megidis).