The oviposition decisions made by insect parasitoids when encountering hosts of variable quality have been the subject of extensive theoretical and experimental investigation. For parasitoids that lay their eggs inside the host, the possible outcomes of encounters with parasitized hosts have been assumed to include only oviposition (superparasitism), rejection, or in some cases feeding on host haemolymph. We document another outcome in Encarsia formosa Gahan (Hymenoptera: Aphelinidae), a species that has been a model system for the study of oviposition behaviour. In E. formosa, females may kill eggs previously laid within the host by jabbing them with their ovipositor before ovipositing themselves. (1) Our observations indicated that jabbed eggs were indeed killed. (2) In experimental arenas in the laboratory, ovicide occurred in the majority of encounters with parasitized hosts and at highest frequency in encounters resulting in oviposition. (3) There was no significant difference in the handling time associated with oviposition+ovicide in parasitized hosts in comparison with oviposition alone, suggesting that there is no time cost to ovicide. (4) Ovicide did not appear to be incidental to normal probing within a host. Radial analysis of the direction of ovipositor movement with respect to the centre of the previously laid egg within the host showed that females engaged in ovicidal bouts probed most often in the direction of the egg. This is the first well-documented study of ovicide in an endoparasitoid. We suggest ovicide may be under-reported in other endoparasitoid species due to the difficulty of observing it. Copyright 2000 The Association for the Study of Animal Behaviour.