Increasing occurrences of blue-green algal blooms in lakes throughout the western United States have been linked to recreational use, sewage inputs, and nonpoint runoff from agricultural and grazing sources. In certain instances these blooms produce toxins that can be lethal to fish, aquatic invertebrates, mammals, and humans. Black Lake in northern Idaho has experienced late summer and fall growths of a toxic alga,Anabaenaflos-aquae. Demonstrated fatal toxicity to cattle and small mammals occurred in 1981, 1982, 1983, and 1985. The eutrophication and subsequent toxic blooms in Black Lake can be related to its large nonpoint nutrient input from the surrounding watershed. The presence or absence of cattle on adjacent meadows may be a major determining factor in bloom formation. Despite a high proportion of sediment (55 percent) exposed to anaerobic conditions during summer stratification, it appears that internal loading alone does not play a significant role in the triggering of a toxic bloom in Black Lake. Development of a toxicA.flos-aquaebloom in Black Lake is dependent on a series of interdependent environmental controls. The most important conditions in Black Lake appear to be high spring nutrient load (compounded by the presence of cattle), high fall water temperature, and stable water column conditions in the fall. TheA.flos-aquaestrain present in Black Lake formed surface concentrations and produced anatoxin-a, despite comprising only 1–2 percent of the total algal biovolume in the water column. Anatoxin-a is produced at times other than when massive surface scums are formed, indicating toxic strains are more widespread in occurrence than previously perceived.