Four hundred and twenty-one isolates of Claviceps purpurea from 38 different host species were cultured on an artificial medium. The conidial cultures were used to inoculate rye, wheat, and barley growing in the greenhouse. All cultures, except one from Glyceria borealis, infected the three host species. Honeydew from rye was used to inoculate 46 gramineous species in the field and in the greenhouse. Every one of these grasses became infected in both locations. It is shown that all indigenous and forage grasses constitute a reservoir of ergot inoculum for rye, wheat, and barley, and that these cereals can be infected provided the environmental conditions are such that inoculum will be disseminated at the time that the cereal crops are in blossom.