Outbreaks of the Douglas-fir tussock moth,Orgyia pseudotsugata(McDunnough), have recurred periodically since 1953 on the Eldorado National Forest in California. The most recent population increase (1974–1978) culminated below an outbreak density, and the number of insects declined again without causing tree defoliation. Details of this culmination and decline were recorded in life tables for the 1978 and 1979 generations. Life processes and mortalities in each life stage were evaluated by k factor analysis for their effects on intergeneration change. The most important factor affecting change was the disappearance of larvae, which accounted for 63% of the total generation mortality. Parasitization of larvae, pupae, and eggs was responsible for 29% and predation of pupae and eggs accounted for 9% of the total. Much of the larval disappearance appeared to be caused by predators, especially arboreal arthropods which commonly preyed on early instars. Although other factors were also involved, these predators apparently played a significant regulatory role by suppressing population buildup in time to prevent serious defoliation.