none Oviposition by the endoparasitoid Tranosema rostrale (Hymenoptera: Ichneumonidae) in larvae of Choristoneura fumiferana (Lepidoptera: Tortricidae) was accompanied by significant changes in total hemocyte counts (THCs) and phenoloxidase (PO) activity of host hemolymph. Three days after parasitization, THCs were half those measured in control larvae and PO activity had undergone a considerable reduction. However, while the parasitoid was able to evade the host's defense reaction, parasitism did not prevent encapsulation of glass rods implanted in 6th instar hosts 3 days following oviposition, although it inhibited melanization of capsules and whole hemolymph. Differential hemocyte counts (DHCs) were also affected by parasitism but the differences were transient. Injection of calyx fluid (CxF), at a concentration of 0.1 female equivalent (FE), mimicked the effect of parasitism on THCs and PO activity, but DHCs were not affected. Injected alone, venom (0.1 FE) had no significant effect on the above variables and, injected together with CxF, it did not modulate the effect of the latter fluid.