Cluster analysis was used to group samples collected from ten stations in Moses Lake, Washington, according to the similarity of their contained algal species. During the period 1968 to 1970, nine recurring distinct sample groups, or algal populations, were identified. Of the nine, three were most distinct; they consistently recurred at the same stations, and were dominated by diatom, green, and blue-green algae, respectively. Of the six species of blue-greens that characterized that population, the recreationally nuisance forms, Aphanisomenon flos-aquae and Microcystis acruginosa, were dominant. The blue-green population was the most wide-spread in the lake and occurred in waters that were warmest and contained the lowest concentrations of inorganic nutrients: nitrogen, phosphorus, and carbon. Green algae dominated in waters that received treated sewage effluent and contained relatively high concentrations of nutrients. As the nutrient content declined proceeding away from that area, blue-green algae became dominant. Temporal variation in biomass (chlorophyll content) of the blue-green population was inversely related to phosphate content, but not to the other nutrients. These results support the hypothesis that nuisance blue-green algae dominate in shallow eutrophic lakes during warm summer months when nutrient content is low because, under these conditions, they apparently out-compete other forms for nutrients. (Author)