The main mortality factors and actions of young larvae of the pine moth, Dendrolimus spectabilis BUTLER were studied by means of artificially introduced population at the experiment station of Tokyo University Forest in Tanashi City near Tokyo. The mortality rate during the first and the second instar always reached 80 to 95 percent of the hatched larvae, and the greater part of mortality seemed to be brought about by the biological factors, such as predators, parasites, and so on. However, the species composition of natural enemies and their actions changed according to observation sites and seasons. It was often observed that predators or parasites disturbed the group of pine moth larvae other than by predation or parasitism and that the mortality rate was increased by their actions.