Dendroctonus ponderosaeHopkins was studied in lodgepole pine,Pinus contortaDouglas, at 4 elevations between 1923 and 2750 m in northwestern Wyoming. The beetle had a 1-year life cycle at 1923 and 2130 m. At 2450 m, part of the population completed a generation in one year, but the remainder required two years. Two years usually were required to complete a generation at 2573–2750 m. Life tables showed high mortality rates and declining populations at the 3 highest elevations, in contrast to high survival rates and increasing populations at the lowest elevation. Cool temperatures at high elevations delayed development, so the beetle overwintered in stages that were particularly vulnerable to winter temperatures. The conclusion is that mountain pine beetle populations are regulated by weather at high elevations.