Host tree diameter is considered an important predictor of the population dynamics of the mountain pine beetle, Dendroctonus ponderosae Hopkins (Coleoptera: Curculionidae: Scolytinae), but the relationship between host tree diameter and beetle traits is unclear. The primary objective of this study was to determine how the quality of individual mountain pine beetles, measured as body size (pronotum width) and lipid content, varied with the diameter of lodgepole pine trees, Pinus contorta var. latifolia Engelmann (Pinaceae). Naturally attacked trees, ranging in diameter from 10 to 35 cm, were selected from stands near Prince George, British Columbia, Canada. Colonisation density and pupal density generally increased with tree diameter, but the number of pupal chambers per gallery start remained constant. Tree diameter positively affected beetle body size, which in turn was positively correlated with absolute lipid content in both sexes and relative lipid content in males. However, tree diameter did not directly predict absolute lipid content, and relative lipid content decreased with tree diameter in males. Larger beetles emerged earlier in the emergence period with relative lipid content remaining constant throughout emergence. All relationships had considerable unexplained variation. Thus, the use of tree diameter or emergence time as predictors of population dynamics of mountain pine beetles should be done with caution.