Specialist ‘plant-ants’ defend their ant-plant hosts from herbivores in exchange for rewards, including shelter and food [1]. Many of these symbiotic associations are obligate mutualisms, in which ant fitness is strongly tied to host protection. Protection should be enhanced by efficient detection of attacking herbivores [1–3]. How information about herbivore presence could be communicated from plant to ant has been little studied. In several systems, plant extracts have been shown to induce increased ant patrolling [2,3], but the compounds eliciting ant defence have never been identified. We have characterized the volatile compounds emitted by damaged leaves of a specialized ant-plant and demonstrated in field experiments the identity of chemicals that induce plant-protective behaviour.