In this study, genus Cosmarium, 3 species, 2 varieties, 1 forma were sampled at 14 stations from August 1987 to July 1988. The character variations in populations were studied from the cultured plants. As a result, 1 species, 1 variety and 1 forma were treated as synonyms according to the polymorphism found at the same colony. C. angulosum f. rotundatum was different from C. angulosum by the front view, but C. angulosum type and C. angulosum f. rotundatum type occurred simultaneously in the same colony. C. angulosum f. rotundatum was included in C. angulosum which is type species. c. auriculatum, C. subauriculatum and C. subauriculatum var. truncatum have been sorted by the shape of cell and the number of granules at the lower sides of semicell. For three types occurred at the same colony, those species and variety treated as synonym of C. auriculatum which was named first.