The marine prasinophycean flagellate Cymbomonas tetramitiformis has been established in culture from Japan and Australia and is examined in detail by electron microscopy. The results are described as a series of reports, the first of which deals with the general cell structure and the structure and morphogenesis of the organic scales that cover the surface of the cell and the flagella. The cell produces seven types of organic scales, all of which are manufactured in the cisternae of the Golgi apparatus. Two scale types are deposited on the cell body and cover the cell in three close-fitting layers, two types are confined to the flagellar pit region, and three types cover the flagellar surfaces. Cymbomonas is quadriflagellate and closely related to another quadriflagellate, Pyramimonas, in scale structure and scale ontogeny. It differs markedly in cell symmetry, swimming behavior, and in details of the flagellar apparatus. The flagellar end of the cell with the flagellar pit is surrounded by a horseshoe-shaped rim, which is very different from the usually four-lobed anterior end of Pyramimonas. Cymbomonas also shows resemblance in scale structure to members of the order Mamiellales, notably Mamiella, and to the phycoma-producing genera Pterosperma and Halosphaera. The Japanese isolate of Cymbomonas formed cysts for a short period, and the cyst appears to be homologous with the so-called phycoma stage of Pterosperma and Halosphaera. Since two chloroplasts were seen in the cysts, the cysts probably arise as a result of sexual reproduction. Concomitant with the presence of cysts in the culture, tiny uniflagellate cells were also present, and they probably represent gametes. Cymbomonas was described from the Adriatic in 1913 and was not reported again anywhere until 1986 and 1987 when Throndsen refound it in the Gulf of Naples in southern Italy. It has subsequently been found in large parts of the world and is now readily recognized by its characteristic scales and mode of swimming. It is often common but does not appear to form blooms.Key words: Prasinophyceae, marine nanoplankton, ultrastructure, Cymbomonas.
