In the East Kootenay region of British Columbia, spruce logs infested by Dendroctonus obesus (Mannerheim) were placed beside thermographs at three sites. Throughout the summer, the mean and minimum air temperatures were higher on a mountain slope than in two valley bottoms at similar or lower elevations. Beetle development was faster on the mountain slope, where it continued until frost occurred in October, at which time 96% of the progeny were mature. In the lower valley bottom the minimum temperature fell 3.9 and 2.8 degrees C (7 and 5 degrees F) below freezing on successive nights in August and larval development stopped. In the valley bottoms only 13 and 9% of the broods matured before winter. Temperature conditions that allow most broods of D. obesus to mature in one season may result in a critical addition to the normal number of beetles that mature after 2 years; development.