none 1. The responses of predators and parasitoids to increased forest tent caterpillar populations were studied by introducing eggs to two trembling aspen, Populus tremuloides Michx., forests where natural populations were at very low density. 2. Of five parasitoid species recovered, only the braconid Aleiodes malacosomatos (Mason) and the tachinid Patelloa pachypyga (Aldrich & Webber) exhibited spatially density-dependent responses, in dry upland forest and mesic lowland forest, respectively. Forest type restrictions on the abundance of these species and the weak density-dependent response by A. malacosomatos suggests that parasitoids may not be capable of regulating low-density tent caterpillar populations. 3. Predation of final-instar larvae and pupae by birds, in particular the northern oriole, Icterus galbula (L.), virtually eliminated the experimental populations. Avian predation was widespread and dominated mortality at all densities. 4. Implications for the initiation of outbreaks of cyclic defoliators are discussed.