The fatty acid composition of filamentous bacterial masses from two very hot Yellowstone Park springs is not unusual despite the extreme environment. Both populations have a series of C14 to C20 straight-chain acids with a maximum at C18, and a series of saturated iso acids with a maximum at C17 in one case and C19 in the other. The fatty acid pattern of this anomalous group of organisms is like that of bacteria but not of blue-green algae. Both populations have similar polar lipids and identical carotenoids. It is speculated that these organisms may be adapted to their high-temperature environment by means of stable lipoprotein membrane systems.