The new genus Prasinoderma Hasegawa et Chihara and the new species P. coloniale Hasegawa et Chihara are described front open water of the western Pacific Ocean as the third coccoid prasinophyte of the marine picoplankton. Its main feature is the presence of one or more layers of thick cell wall without any appendages on the cell surface. Asexual reproduction is by bipartition and a new cell wall is formed around cach of the two daughter cells. One daughter cell is released while the other retains the parent wall. This type or reproduction is also known in Prasinococcus Miyashita et Chihara, tentatively placed in the family Pyenococcaceae (Mamiellales, Prasinophyceae). During the present study, it was found that Pyenococcus provasolii Guillard reproduces by a type of asexual reproduction essentially similar to that of Prasinococcus and Prasinoderma. The circumscription of the family Pyenococcaceae is emended to adopt this characteristic method of reproduction. Thus, the family currently accommodates three genera, Pyenococcus, Prasinococcus and Prasinoderma.