The clinical signs of fescue foot mimic those of ergotism in cattle, a disease caused by ergopeptine alkaloids of ergot (Claviceps purpurea) sclerotia. Although several other types of alkaloids have been detected in tall fescue, ergopeptine alkaloids have not been previously detected. Ergovaline (the most abundant alkaloid of this class in fescue), ergosine, and lesser amounts of other ergopeptine alkaloids were detected by MS/MS in two tall fescue pastures. Both pastures were infected with the endophytic fungus Epichloe typhina, and both produced clinical signs of fescue foot in cattle. The total concentration of ergopeptine alkaloids present in the fescue pasture samples was 0.2 ppm and 0.6 ppm, respectively. The profile of ergopeptine alkaloids found, together with the absence of ergot sclerotia in the pastures, indicates that the endophytic fungus is the source of these alkaloids.