Mycoviruses or fungal viruses have frequently been reported from fungi and are often associated with symptomless infections. Several studies have demonstrated that viruses regulate hypovirulence in pathogenic fungi. However, the effect of mycoviruses on mutualistic interactions, especially in natural ecosystems, is practically unknown. We are investigating the biodiversity and ecology of endophytic fungi and their mycoviruses from a dominant grass, big bluestem (Andropogin gerardii) collected from 2004 - 2006 from plots periodically burned at the Tallgrass Prairie Preserve, Pawhuska, OK. This study showed that the root fungal endopyte community is very diverse: most fungi are from the classes Dothideomycetes and Sordariomycetes, the predominant fungal genera were Perconia, Gaeumannomyces, Fusarium, Anguillospora, and many unknown species. We found that many of the fungi are infected by viruses. Since the majority of these fungi have no previously reported viruses, these viruses may be completely new.  In future studies, we will aim to understand the role of these mycoviruses on the character and intensity of plant-fungus interactions and possibly how disturbances, such as fire, can influence virus-fungal-plant interactions.