||The parasitism of tall fescue by an endophytic fungus is reviewed relative to the biology of the fungus and the symptomless expression within the grass. A consideration of the taxonomy of the fungus Acremonium coenophialum is presented and justifications for the affinity of the fungus with the clavicipitacious fungus tribe Balansiae is discussed. The in vitro physiology and growth of the fungus in culture is reviewed and related to fungus specific compounds, as well as compounds specific to the association. The ultrastructural morphology of the association, possible growth and survival benefits of infected grass, and data on insect and cattle toxicities suggest that the fungus and fescue formed a symbiotic relationship early in evolution. The economic advantages and future biotechnological uses of this endosymbiotic system, as well as other new combinations of grass species with endophytes, suggest a new and exciting era in forage, turf, and conservation grass development programs.