1. Entomopathogenic nematodes (EPN) are currently being used as introduced biological control agents against the larvae of the native European forestry pest Hylobius abietis L. which develop under the bark of stumps and roots of newly dead conifer trees. 2. The potential for resource competition between gregarious ectoparasitoid Bracon hylobii Ratz and EPN by recording oviposition and related behaviours of B. hylobii females on EPN-infected H. abietis larvae was investigated. Wasps did not parasitise EPN-infected host larvae that were dead when presented, but naïve and experienced wasps parasitised live EPN-infected hosts. Naïve wasps parasitised live EPN-infected hosts significantly less frequently than healthy hosts only when the infected larvae were close to death (i.e. died during 24-h trial). Parasitism by experienced wasps was unaffected by host infection. 3. Wasp probing and oviposition were positively associated with the amount of host movement. Preventing H. abietis larvae from chewing on bark significantly reduced parasitism by naïve, but not experienced wasps. 4. The number of eggs per clutch was not affected by bark chewing or EPN-infection of H. abietis larvae. 5. Naïve and experienced B. hylobii parasitised two abnormal hosts (larvae of coleopteran Rhagium bifasciatum Fabricius and lepidopteran Galleria mellonella L.), both of which moved and chewed on bark during trials. 6. It was concluded that B. hylobii can use vibrational cues generated by host movement and feeding to locate hosts at short range and accepts unsuitable (EPNinfected or abnormal) hosts as long as these create such cues. The implications for competition between B. hylobii and EPN and possible ways of minimising it when applying EPN are discussed.