Observations on thermophilic algal communities during a three‐year period at Mount Rainier National Park and during January 1965 and 1966 in Yellowstone National Park have been analyzed to evaluate the distribution of species in thermal streams relative to temperature and competitive interaction. Although temperature appears to be the most important physical factor affecting the distribution in alkaline springs, interspecific competition for space also influences the distribution. Diatom species commonly occurring above 35C are noted, along with the apparent tolerance by certain species of temperatures in excess of 45C. The diversity of the diatom flora increases as the water cools. Despite thermal and chemical constancy, the growth rate of the algae is negligible during the winter months when light is minimal.