The thick mats which develop in alkaline hot springs of Yellowstone National Park in the temperature range 50–70 C contain only a thin surface layer of algal material, exclusively of the genus Synechococcus. All of the underlying filamentous material, which resembles species of Phormidium, is shown to be devoid of chlorophyll or photosynthetic ability, and probably consists of members of the Flexibacteria. In the flowing water of these springs, a layer of filamentous bacteria first develops and only then does the unicellular Synechococcus appear, apparently becoming established in the matrix formed by the filamentous organisms. The above conclusions are supported by data from quantitative chlorophyll extractions, fixation of 14CO2, fluorescence microscopy, autoradiography, and the use of experimental channels.