Yearly trends of lodgepole pine (Pinus contorta var. latifolia Engelm.) mortality in relationship to attacking and emerged mountain pine beetle (Dendroctonus ponderosae Hopkins) populations were measured during the life span of an epidemic within a square-mile area. Epidemic levels of infestation were recorded for 7 yr, with peak tree killing occurring the 4th yr. More small trees were killed than large ones, but in proportion to their occurrence the large trees were killed 1st and at a higher rate. Both density and total numbers of attacking and emerged beetle populations exhibited marked changes during the course of the epidemic. The patterns of total attacking and emerging beetle populations were similar to the trend of annual tree mortality, but population densities expressed on a square-foot basis showed different trends. Generally, the density of attacking beetles increased during the epidemic, and while emergence remained relatively stable during the build-up years, it declined rapidly during and following the peak year of tree killing. Up to and including the year of maximum tree killing, trees of all sizes produced more beetles than they absorbed, but approached or fell below a hypothetical 1:1 emergence-attack ratio during the waning years of the outbreak. A greater percentage of emerged beetles became established in host trees during the build-up and peak year of the infestation than during the declining years.