1. Among Zostera and Phyllospadix there are found in Japan 7 species satisfactorily identified, of which 4 are endemic, viz: Zostera asiatica, Zostera cacspitosa, Phyllospadix japonicus and Phyllospadix iwatensis. 2. Following 3 species and 1 variety may be reduced to synonym or form of another one:Zostera japonica is a brackish form of Zostera nana ROTH. Zosetra pacifica is a synonym of Zostera asiatica MIKI, Zostera marina var. stenophylla is a form of Zostera marina L. Phylloepadix Scouleri of Japan is a synonym of P. iwatensis MAKINO. 3. There are two types of rhizome, viz. sympodial and monopodial, and their flowering shoot may be divided into three types. Each terminal spadix is formed originally as a pseudoterminal one. 4. The seedling of Zostera nana has a primary root and its terminal creeps on the soil. 5. The seedling of Zostera marina is provided with a flowering shoot on its terminal in the same year and passes the winter by the lateral shoot. 6. Rhizome of Zostera marina branches only in the winter season. The growth of each node takes neary half a month. 7. Zostera and Phyllospadix in Japan may be divided into three groups according to their geographical distribution, viz, that of wide range, cold current and temperate sea. 8. Zostera are found on muddy or sandy places of the bay, while Phyllospadix upon the rocks washed by open sea water. 9. Zostera asiatica and Z. caulescens are found in deeper places than the other.