Laboratory experiments using a glass tube arena were conducted to determine the effects of aggressive behaviour of the stemborer, Chilo partellus Swinhoe (Lepidoptera: Pyralidae), on oviposition success and mortality of the parasitoid, Cotesia flavipes Cameron (Hymenoptera: Braconidae). On contacting unparasitised hosts, female parasitoids rapidly oviposited and then retreated from the experimental arena. Hosts exhibited no aggressive behaviour prior to oviposition. However, soon after being attacked, host larvae became more aggressive, spitting at and biting the parasitoids. When Co. flavipes females were introduced toward the head of the stemborer, the parasitoids stung the host near the head and were frequently bitten and killed by the hosts. Parasitoids that were introduced to the host from the posterior end of the abdomen, stung the abdomen and were often able to escape from the host without being bitten. Regardless of whether parasitoids were introduced near the head or the abdomen, they were more often killed by older hosts than younger hosts. Parasitised hosts were more aggressive than unparasitised hosts one hour after the initial parasitisation, but not after 24 hours. Larval saliva of Chilo partellus was found to reduce survival of adult female parasitoids. The aggressive behaviour of Ch. partellus was not effective as a self-defense against Co. flavipes, but is likely to be an important mortality factor of Co. flavipes in nature.