ABSTRACTThe UTEX 2193 strain ofScenedesmus armatus(Chod.) Chod, when cultured in any of several media (whether natural or artificial, concentrated or dilute) produced a variety of colonial morphologies as well as a unicell population. Morphological expression was related to culture ape. When the initial cell density was just a feu1 hundred cells per mL. the culture first produced a unicell population, then spiny colonies, and as stationary phase was approached, spine‐less colonies.Two classes of spiny colonies were detected. Type I colonies had elongate cells with the terminal cells shorter than median cells. Spines were longer than cell length. The wider, oval, grainy cells of Type II colonies were uniform m length. Spines were shorter and thicker than those on Type I colonies. Only Type I colonies produced unicells: the latter appeared as two morphs. The smaller unicell was obovoid with four delicate spines: the larger had ovate cells bearing four thicker spines.Control of unicell development in all media was achieved by carefully monitoring colony type and cell number used for the inoculum. A unicellular population developed in batch culture in defined media, both concentrated and dilute, when the initial cell density (either Type I or Type II colonies) was low (below 1000 cells‐mL−1), as well as in synchronous cultures. With higher initial cell densities, e.g. 2 × 104cells·mL−1, the inoculum had to contain Type I colonies to produce unicells. Unicells were also produced in water from Agronomy Pond, where the strain originated. We discuss the role of unicell populations in the distribution ofSc