Many grasses are infected by systemic fungal endophytes (family Clavicipitaceae, Ascomycetes) that produce pysiologically active alkaloids in the tissues of their hosts. Infection makes grasses toxic to domestic mammals and increases resistance to insect herbivores. Some grasses are sterilized by endophyte infection while remaining vegetatively vigorous; other infected grasses remain completely fertile. Experiments demonstrate that plant growth and seed production can be increased by infection. This symbiotic association may be a defensive mutualism in which the fungi defend their hosts against herbivory, thereby defending their own resources. Recent studies suggest that defensive mutualism of endophytes with grasses may be widespread.