The mountain pine beetle infestation has been epidemic in lodgepole pine in Yellowstone National Park since 1966. Infestation boundaries have advanced steadily northward and eastward. The infestation now encompasses nearly one-half of the total Park area. Surveys indicate a decline in tree mortality the last 2 years. In 1970, an average of 18.9 lodgepole pine was killed per acre. In 1971, 19.0 were killed per acre. In 1972 and 1973, these figures declined to 16.7 and 6.6 trees per acre respectively. Average diameter of attacked trees has decreased from 12.0 inches d.b.h. in 1971 to 10.0 inches d.b.h. in 1973. A decrease in number of infested trees occurred in older infestation centers in the southwest corner of the Park due to depletion of available hosts.