Dendroctonus jeffreyi and D. ponderosae are sibling species of bark beetles (Coleoptera: Scolyti- dae) with few morphological and molecular genetic differences. The two species are believed to have di- verged relatively recently. Dendroctonus jeffreyi colo- nizes only Pinus jeffreyi, while D. ponderosae colonizes up to thirteen Pinus spp., but not P. jeffreyi. Adult beetles of both D. jeffreyi and D. ponderosae carry sym- biotic fungi in mycangia located on the maxillary car- dines. Dendroctonus ponderosae was known to carry two fungi, Ophiostoma clavigerum and 0. montium, in its mycangia. However, it was not known which fungi might be carried by D. jeffreyi. Fungi were isolated from the mycangia of over 900 D. jeffreyi collected from a large portion of its geographic range. Using morphology, isozyme phenotypes, and growth rates at different temperatures, all isolates from D. jeffreyi mycangia were determined to be 0. clavigerum; 0. montium was never isolated from D. jeffreyi mycangi
