none This study explores within-host mechanisms of intra- and interspecific competition involving the gregarious braconid endoparasitoid, Apanteles glomeratus, and the granulosis virus (GV) of Pieris brassicae. Increasing doses of the GV resulted in increases in mortality and decreases in lifespan of the host. Susceptibility to the virus decreased markedly with host age. High doses of the GV resulted in substantial reductions in production of pathogen progeny, consistent with intraspecific competition for a limited (but growing) resource. Survival of the parasitoid brood within GV-infected hosts depended on the relative timing of parasitoid emergence (at the spinning of the host pupal mat) and host death due to the virus. Interspecific competition within the host was, on average, highly symmetrical. The presence of virus reduced the average weight of the parasitoid brood by 29%, whereas the parasitoid brood reduced the potential reproductive output of the virus by 28%.