Autotrophic growth yields of four strains of Sulfolobus using tetrathionate as sole energy substrate fell in the range 6.2-7.8 g dry weight (tool tetrathionate oxidized) -1. Autotrophic organisms lacked ribulose 1,5-bisphosphate carboxylase, but contained pyruvate and phosphoenolpyruvate carboxylases. S. brierleyi and strains B6-2 and LM exhibited mixotrophic growth, with tetrathionate oxidation, COz-fixation and organic substrate assimilation occurring concurrently, using media containing glucose or acetate. Yeast extract or succinate supported heterotrophic growth and showed strain-dependent repression of one or both of tetrathionate oxidation and CO2-fixation resulting in biphasic growth. All four carbon atoms of succihate were assimilated to cell-carbon during growth. Acetate was the major source of cell-carbon during mixotrophic growth. These observations are not inconsistent with the possibility of a reductive carboxylic acid cycle in these organisms. Radiorespirometric analysis of glucose oxidation indicated CO2 release to occur by means of an Entner-Doudoroff pathway (followed by pyruvate decarboxylation) and oxidative pentose phosphate pathway reactions. There was little evidence from the glucose radiorespirometry of the large-scale use of an oxidative tricarboxylic acid cycle for terminal oxidation of acetate derived from pyruvate. These results demonstrate the considerable metabolic versatility of Sulfolobus strains and show that there is significant variation among them.