Ergot, a fungus that infests cereal plants, notably rye. It forms black, dense masses called sclerotia, which replace many of the host plant's seeds.

In areas where rye is the chief source of food, epidemics of the disease ergotism, or St. Anthony's fire, have developed among persons who ate bread made of contaminated rye flour. In this disease, the smaller arteries carrying blood to the feet and hands are constricted (made smaller) by the action of ergot. This constriction cuts off the blood supply, resulting in death of tissue. Gangrene easily develops in such tissue.

Several useful drugs are made from sclerotia. These include ergotamine, prescribed for migraine headache; and ergonovine, administered to mothers immediately after childbirth to prevent hemorrhage.

Ergot make up the genus Claviceps. The most common species is Claviceps purpurea.