Mountain pine beetle infestations in second-growth ponderosa pine developed in 1973 in the Little Rocky Mountains. In 1976, increasing numbers of attacked trees were observed. Current infestation intensity averages 7.1 trees per acre. Losses are expected to continue as long as stands remain overstocked and stagnated. Reduction of basal area by commercial sales and/or thinning is recommended. Precautions are given to prevent population buildup of secondary bark beetles.