The prasinophycean flagellatePyramimonas orientalis has been examined by light and electron microscopy of wild and cultured material. The many different scales which cover all cell surfaces, including the flagella, are described; their synthesis and assembly in the two Golgi bodies have been examined. The Golgi bodies work simultaneously to produce all-at least five—scale categories, including hollow hair shaped scales. From the Golgi system the scales become transported to a special container—a reservoir—in which they, in an unknown way, separate and become arranged in the same pattern as on the body surface. From the reservoir, the scales move through a duct to the cell surface, apparently together with the subtending membrane, which thus becomes incorporated in the plasmalemma or the flagellar membrane. The liberation process, which differs from that of other species ofPyramimonas examined, is illustrated diagrammatically, starting at two extensions of ER from the nuclear envelope.The flagellar apparatus possesses a flagellar root system of the green algal type, a finding of phylogenetic significance. Furthermore, near the flagellar transition region a structure was observed, which at present is known from certain “brown” groups of algae, but never from any green flagellate. The taxonomic implications are discussed briefly, and a virus attacking the nuclear area of the cell is reported. Very surprisingly two different sizes of the virus were found, which may be different stages of the same “organism”.