Data were collected during 2 consecutive years on the Starkey Experimental Forest near La Grande, Oreg., to examine phenology of the western spruce budworm,Choristoneura occidentalisFreeman. The logistic probability distribution was used to describe distribution of budworm in each developmental stage (instar 2 through adult) as a function of accumulated degree-days by year, plot, and host species. The logistic distribution described phenology accurately and was simpler to compute than other methods. Results should interest insect-pest managers and researchers who model development of plants and animals.