A direct relationship between alkaloid synthesis and conidia formation was observed in a series of mutants of the saprophytic fungus Claviceps purpurea 129 which produces high quantities of clavine alkaloids (4500 microgram-ml-1). Sclerotia-like mutants forming mostly arthrospores and only a low number of conidiospores were exceptional in that they were characterized by an inverse relationship between alkaloid synthesis and conidial formation. Conidiospores of the parent strain 129 differed both in their size and ultrastructure. They include oval microconidia (2 X 3.5 micrometer) and prolonged macroconidia (2 X 10.5 micrometer). Both conidial types originated on short lateral sporophores terminated by phialids. Most conidia were characterized by electron-dense cytoplasm with a number of lipidic inclusions; a minority of conidia contained granular cytoplasm and vacuoles with membrane invaginations. Bicellular conidia were only rarely observed.