Microbiological examination of prairie grass roots usually reveals the presence of diverse mycorrhizal and dark septate endophytic (DSE) fungi. Some of these fungi can enhance plant resistance to abiotic stress in extreme environments. The host specificity of two DSE fungi isolated from pure stands of crested wheatgrass and Russian wildrye in southwest Saskatchewan was assessed in microcosms. Pure cultures of endophytic fungal isolates were obtained after plating surface sterile root pieces on Potato dextrose agar. Plugs containing these fungi were used to individually inoculate germinated surface sterile seeds of crested wheatgrass and Russian wildrye, and control plants were mock inoculated with sterile agar plugs. The colonization of crested wheatgrass and Russian wildrye roots was 4.5 and 20 times higher by the DSE fungi isolated from the same plant specie, 75 days after inoculation. No colonization was seen in the control plants. Both DSE fungal isolates were non-specific colonizers, but exhibited host preference. The molecular identification and phylogenetic analyses of these fungi are under way.