Abstract: The West Lake is an eutrophic lake abundant with populations of blue-green algae. A survey in September and November 1980 indicated that the phytoplankton in the lake displayed vertical migrations between water layers twice a day. A bi-peak pattern of diurnal variation in algal density appeared regularly or irregularly in the 0.5 meter water layer. Similar but less apparent rhythms occurred in the deeper layers. The peaks usually occurred about 2 hours after sunrise and sunset. It was suggested that the diurnal rhythm of the algal populations was closely related to the alternation of light and dark periods and the buoyancy regulation of the dominant species.