Triradiate forms of the heterothallic desmid Cosmarium botrytis Menegh. and the homothallic C. botrytis var. tumidum Wolle have been isolated and grown in clonal culture. Although biradiate cultures are stable, triradiate cells are sometimes formed by the products of germinating zygospores. The triradiate cultures were found to be fully fertile, but the character is not transmitted sexually. Mechanisms for the formation and vegetative perpetuation of the triradiate character and for its gradual, but irreversible, change back to the biradiate form are discussed.