A new genus, Prasinococcus, and a new species, P. capsulatus, are described on the basis of specimen that appeared in enriched cultures inoculated from water samples in the western Pacific Ocean. Cells of this unicellular alga are spherical and surrounded with a large amount of gelatinous matrix. The pigment composition, which includes chlorophylls a and b, prasinoxanthin, Mg 2,4-divinylphaeoporphyrin a, monomethyl ester (Mg 2,4-D) and 5,6-epoxy-3,3'-dihydroxy-5,6,7',8'-tetrahydro-beta-epsilon- caroten-11',19-olide (uriolide), is close to that observed in the Mamiellales (Prasinophyceae). The cell is non-flagellate and has a firm cell wall, but no scales on the cell body. The pyrenoid has a characteristic structure in which a mitochondrial lobe and chloroplast outer membranes protrude into the pyrenoid matrix. The cell wall has a projecting appendage like a circle collar surrounded by distinctive holes penetrating the cell wall. Since the cell contains no 3-deoxy-manno-octulosonic acid (KDO), the wall is considered to have a different origin from the cell coverings which consist of fused scales in Tetraselmis species. It has a characteristic way of asexual reproduction in which, after cell division, one cell remains within the parent cell wall while the other is extruded. This suggests that this alga be placed tentatively in the Pycnococcaceae (Order Mamiellales) on the basis of the pigment composition and morphological characteristics, but the taxonomic position of coccoid prasinophytes may need to be reconsidered.