Paleoecological reconstructions from two lakes in the U.S. northern Rocky Mountain region of Idaho and Montana revealed the presence of bark beetle elytra and head capsules (cf. Dendroctonus spp., most likely D. ponderosae, mountain pine beetle). Occurrence of these macrofossils during the period of time associated with the 1920/1930 A.D. mountain pine beetle outbreak at Baker Lake, Montana suggest that when beetle populations reach epidemic levels, beetle remains may be found in the lake sediments. In addition to the beetle remains found at Baker Lake during the 20th century, remains were also identified from ca. 8331, 8410, and 8529 cal yr BP. At Hoodoo Lake, Idaho remains were found at ca. 7954 and 8163 cal yr BP. These Holocene records suggest the infestations occurred during a period when climate changed rapidly to cooler and effectively wetter than present in forests dominated by whitebark pine. These two lake records provide the first preliminary data for understanding the longterm history of climatic influences on Dendroctonus bark beetle activity, which may be useful for predicting climate and stand conditions when mountain pine beetle activity occurs.