The desirability is discussed of a subdivison of the genus Ceratocystis into a group characterized by the presence of a phialidic conidial state, and a group in which the conidia are usually produced exogenously. Corroborative evidence for this arrangement is found in the carbohydrate constitution of the cells: in all examined species of the former group (Ceratocystis s. str.) rhamnose and cellulose are absent, whereas in the latter both components are present (Ophiostoma H. et P. Sydow).