Quadriflagellate zoospores from an isolate ofCylindrocapsa geminella have been studied ultrastructurally. Each swimming zoospore is enclosed within a delicate, loose-fitting, outer investment which in grazing tangential section exhibits a pattern resembling wire gauze. The chloroplast is axial and possesses numerous, irregularly shaped lobes, each joined to the central pyrenoid region by a slender connection. A single pyrenoid per zoospore is typical, but as many as 3 per cell have been observed. The pyrenoids have a distinctive structure; the matrix is penetrated by branched cytoplasmic channels delimited by a double membrane system continuous with the chloroplast envelope. Formation of new pyrenoids by bipartition is common. Each zoospore has an eyespot consisting of a single layer of closely packed pigment chambers positioned directly beneath the chloroplast envelope near the cell surface. Contractile vacuoles are prominent near the flagellar bases and throughout their development they are associated with numerous coated vesicles; membrane continuity is often observed between the vesicles and the contractile vacuole. Subunits on the surface of the coated vesicles appear identical to similar subunits occurring on the outer surface of both expanding and fully expanded contractile vacuoles. Preliminary observations are given on the structure of the flagellar apparatus.