SUMMARYThe Ultrastructure ofMicrothamnionzoospores is described (exclusive of the finer details of the flagellar apparatus). The zoospores have a typical chlorophycean morphology but, in addition, many unique features. The chloroplasts contain starch but no pyrenoid. Thylakoids may run from one edge of the chloroplast to the other and usually anastomose into 2‐ to 8‐membered stacks. The internal morphology is highly polarized and characterized by an intimate proximity and constant spacing between many organelle membranes. All the organelles are asymmetrically distributed within the cell in a precise manner. The anterior region of the zoospore is attenuated into a neck which contains a single, massive mitochondrion. A fibrous rhizoplast lies beside the mitochondrion and appears to connect the flagellar apparatus directly with the outer membrane of the nuclear envelope. In addition, this outer membrane is extended over a distance of several microns to eventually lie in close proximity to the basal bodies. Oil vacuoles and lipid bodies are restricted to the posterior end of the zoosp