In order to investigate the mortality factors regulating the pine moth, in areas with low population densities, trap studies with laboratory reared larvae have been conducted in various biotopes in Sweden, and in old recorded outbreak areas in western Sweden and Norway between 1978-82. A total of 15 parasitoid species have been found, among them 8 primary- and 7 hyperparasitic wasps. The most important regulating factors have been two gregarous ectoparasitoids, Iseropus stercorator Fabr. and Gregopimpla bernuthii Hartig (Hym., Ichneumonidae, Pimplinae), which parasitize the prepupae of the host. The parasitizing frequency of these two parasitoids ranged between 5-80% in the various biotopes.