The phenology of bud burst and shoot elongation of the host tree and egg hatch and larval development were studied on 3 areas in a 1972 infestation. Bud burst and egg hatch were found to be closely related to accumulated degree-days, and peak egg hatch occurred when 77–97% of the buds had burst. Larval development then closely followed shoot elongation. This synchrony of host and insect phenology provides an easily observed field event for monitoring Douglas-fir tussock moth development.