Apanteles kariyai Watanabe (Hymenoptera, Braconidae) is successfully able to parasitize Pseudaletia separata (Walker) (Lepidoptera: Noctuidae) until 4 days before pupation. The function of the substances (calyx fluid and venom fluid) injected at the time of oviposition seems to be important. The fat body functions as an important energy source for the host larvae to pupae as well as for the parasitoid. The fat body in Pseudaletia separata parasitized by Apanteles kariyai sequestered no protein granules, while the control larvae in the same day stored many protein granules in trophocytes. The protein concentration in hemolymph of the parasitized host showed low level compared with that of the control larvae. These phenomena were reproduced on the host in case of simultaneous injection of calyx and venom fluids. Both calyx and venom fluids seem to regulate the physiological phase of host in early stage of the parasitization. none