Mortality from various causes was recorded in a Douglas-fir tussock moth, Orgyia pseudotsugata McD., outbreak southeast of Coeur d'Alene, Idaho, in 1974. Observations began June 27 when about 50 percent of the larvae were second instars and continued until pupation. Within a 35-day period, there was a 93 percent average reduction in population. Approximately 36.7 percent of the population collected and reared in the laboratory died from unknown causes; 11 percent were parasitized; and 3.4 percent were killed by nucleopolyhedrosis virus. Phobocampe n.s. was the most abundant parasite and attacked all instars. Unlike most other past outbreaks, this one did not collapse at the end of 3 years due to virus and parasites. Egg masses were found in infested areas during the fall of 1974.