Field and laboratory studies were used to examine the differential effects of the two primary filamentous fungal associates of Dendroctonus ponderosae (mountain pine beetle) on beetle fitness and development time in western Montana. In a field study, size, lipid content and exit date of adult beetles emerging from trees carrying Ophiostoma clavigerum were compared to those carrying O. montium. In a laboratory study, phloem sandwiches were used to experimentally manipulate the fungal complement that different life stages of the insects developed with. The results of both studies will be presented and the potential implications of these symbiotic interactions on the population dynamics of this ecologically and economically important bark beetle discussed.