The green alga Monoraphidium sp., isolated from a pond at Changhua city in Taiwan, was examined for its morphological responses to different environmental factors. The alga exhibited fusiform and spherical cells when cultivated in the laboratory. Three different temperatures (20°C, 25°C and 30°C), adjustments in the initial pH of medium from 6.22 to 9, and replacement of the medium at late-stationary phase on day 25, were tested to determine the factors that affected the morphotype alternation in this green alga. In these physiological experiments, fusiform cells grew flourishingly in fresh medium and transformed into spherical cells in aged medium. Spherical progeny were observed in all physiological experiments. In the temperature experiments, the size of spherical cells at 30°C was significantly larger than that of those at 20°C and 25°C (P < 0.05). Through time, medium alkalization occurred naturally in the temperature experiments. However, temperature and pH were observed to have no significant influence on morphotypealternation pattern (P > 0.05). Replacing the medium at late-stationary phase resulted in drastic alternation between fusiform and spherical morphotypes. This result suggests that medium content is the major factor influencing morphotype alternation. Based on these experiments and detailed observations, a schematic model of morphotype alternation in Monoraphidium sp. is suggested.