A generally accepted hypothesis is that lodgepole pines infected by dwarf mistletoe,Arceuthobium americanum, are less susceptible to mountain pine beetle,Dendroctonus ponderosae, because they have thinner phloem than uninfected trees. This Colorado study, based on 1,051 trees, indicates that there is little relationship between dwarf mistletoe intensity and phloem thickness. Therefore, we conclude that there is little correlation between dwarf mistletoe and mountain pine beetle activity, at least in Colorado.