Venom from the pupal endoparasitoid Pimpla turionellae L. (Hymenoptera: Ichneumonidae) contains a mixture of biologically active components, which display potent paralytic, cytotoxic, and cytolytic effects toward hosts. Here, we further investigate whether parasitism or envenomation by P. turionellae alters hemocyte numbers of its host Galleria mellonella L. (Lepidoptera: Pyralidae). Total hemocyte counts declined sharply in pupae and larvae of G. mellonella exposed to P. turionellae. These same cellular responses occurred when wasp venom was artificially injected into hosts, suggesting that venom alone induces cytotoxicity in hemocytes. Analysis of the differential hemocyte counts in untreated pupae and larvae revealed that more than half of the circulating hemocytes were granular cells followed by plasmatocytes. Parasitism reduced the number of granular cells while increasing the number of plasmatocytes. This trend was most evident at 4 h postparasitism, and a similar trend was observed with the artificial injection of high (but not low) doses of venom. When isolated larval hemocytes were exposed to a LC₉₉ dose of venom, a differential response was observed for granular cells versus plasmatocytes. Both types of cells displayed some formation of vacuoles within the cytoplasm within 15 min posttreatment. However, the degree of vacuole formation was much more extensive in granular cells at later time points than for plasmatocytes, and granular cells seemed much more susceptible to venom as evidenced by cell death.