Thirty seven dinoflgellate species are described from the sample water collected from August 1998 to July 2000 at Lake Nakaumi, brackish lake, west Japan. The assemblage consists of three main groups, Prorocentrum minimum, Heterocapsa rotundata and small Gymnodinium complex. These dinoflagellates are present in the lake all the year around. They are adapted themselves to intense environmental changes of brackish water lake.