The host suicide hypothesis1, which derives from inclusive fitness theory2, postulates that parasitized individuals in spatially aggregated populations consisting of close kin may actively enhance their probability of dying. The fitness cost associated with suicide becomes negligible when infection by a parasitoid causes the expected reproduction of the host to approach zero. But the host will benefit from suicide, if by its death (and that of its parasite) the level of subsequent parasitism in its kin is reduced relative to that in non-kin. Although conceptually appealing, host suicide has not yet been clearly demonstrated3. Here we report that pea aphid (Acyrthosiphon pisum) parasitized by the braconid wasp Aphidius ervi, exhibit apparent suicidal behaviour in response to both aphid alarm pheromone and approaching coccinellid (ladybird beetle) predators. We believe this to be the first convincing evidence in support of the host suicide hypothesis.