Plants of Festuca versuta contain a fungal endophyte in leaf sheaths, stems, seeds, embryos, and seedlings. The endophyte was isolated and its cultural characteristics were compared to those of the endophytes of F. arundinacea and Lolium perenne. The endophyte of F. versuta appeared to be distinct from endophytes of other forage grasses. On the other hand, the endophytes of all three grasses produce an antifungal substance(s) in vitro, which may represent a common feature of these microorganisms in vivo.