The endoparasitic wasp Pimpla turionellae L. (Hymenoptera: Ichneumonidae) injects its pupal host with venom during oviposition. Venom from P. turionellae has previously been shown to contain a mixture of biologically active components, which display potent paralytic, cytotoxic and cytolytic effects towards lepidopteran and dipteran hosts. This study was undertaken to investigate if parasitism and/or envenomation by P. turionellae affects the encapsulation and melanization responses of its host Galleria mellonella L. (Lepidoptera: Pyralidae) in larval and pupal stages. Analysis of the effects of venom on encapsulation and melanization of the Sephadex A-25 beads revealed that the number of beads strongly encapsulated and melanized were reduced by more than 50% at 4 and 24 h post-venom injection into pupae. Injection of a lethal dose of venom (0.5 venom reservoir equivalent) in the last instar larvae was sufficient to reduce the ability of haemocytes to encapsulate the beads by more than 50% at 4 h post-injection. Similar results were also obtained when beads were recovered from parasitized pupae indicating that parasitization by P. turionellae suppressed haemocyte-mediated encapsulation in G. mellonella. We found that the cellular defence reactions occur more rapidly in larvae compared with pupae of G. mellonella, indicating the higher susceptibility of pupal haemocytes to parasitism and venom injection.