Epichloë typhina (Ascomycetes:Clavicipitaceae) is an endophytic fungus which grows symbiotically within several grass species of the subfamily Pooideae, including Dactylis glomerata, orchardgrass. E. typhina is heterothallic; it must out-cross gametes in order to produce ascospores, propagules which can infect new hosts. Previous studies have described a tightly linked mutualism between Epichloë spp. and Botanophila lobata (Diptera:Anthomyiidae), a fly which facilitates out-crossing of endophyte gametes during an elaborate egg-laying behavior. Upon hatching, larvae develop to the pupal stage on a single stroma, feeding on the fertilized fruiting body. E. typhina, D. glomerata and B. lobata are all non-native to the Willamette Valley, Oregon, but now exist widely throughout the region. How do closely co-evolved organisms interact when introduced outside of their native context? During the 2008 growing season, observations of the fly-fungus-grass system were recorded in both cultivated and naturalized areas. The spatial variability of fly and endophyte association regarding fertilization rate and overall endophyte incidence will be presented. The applicability of mutualism as a description for fly-endophyte association will be discussed.