Aerial surveys of the Nezperce National Forest in Idaho revealed 138,692 acres of aerially visible top kill and tree mortality due to repeated defoliation by western spruce budworm, Choristoneura occidentalis Freeman. A small ground sample indicates that up to 47 percent of the grand fir volume was affected by top kill in some areas. True firs were most severely affected by repeated defoliation, and Engelmann spruce was intermediately affected. No top kill or tree mortality was observed on Douglas-fir.