Larval endoparasitoids can avoid the immune response of the host by the function of polydnavirus (PDV) and venom. PDV infects hemocytes and affects the hemocyte function of the host. In this paper, we investigated how PDV and venom affect the hemocyte population of the host. Cotesia kariyai, the larval endoparasitoid, lowers the hemocyte population of the noctuid host larvae soon after parasitization. The reduction in the number of circulating hemocytes is caused by the breakdown of the circulating hemocytes and of the hematopoietic organ which generates the circulating hemocytes. The decrease in the number of hemocytes shortly after parasitization is a response to the venom. However, the decrease in hemocyte population on and after 6h post-parasitization appears to be caused by the PDV. Apoptosis in circulating hemocytes was observed on and after 6 It post-injection of PDV plus venom. It was revealed through cytometry that mitosis of circulating hemocytes was halted within 24 h after the injection of PDV plus venom. Apoptosis in the hematopoietic organ was induced 12 It after the injection of PDV plus venom. Furthermore, the plasma from the hosts injected with PDV plus venom depressed the number of hemocytes released from the hemotopoiteic organs. [copyright] 2004 Elsevier Ltd. All rights reserved. Pseudaletia separata (Noctuidae) : Host Cotesia kariyai (Braconidae) : Parasite Circulatory system; Haemolymph; Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts; Immunology and repair mechanisms Cotesia kariyai; Lepidopteran hosts; Mechanism of reduction in number of circulating haemocytes; Pseudaletia separata Pseudaletia separata; Haemocytes; Hymenopteran parasites; Mechanism of reduction in number of circulating haemocytes; Cotesia kariyai; Immune like reactions none