Growth and hydrogen production by two extreme thermophiles during sugar fermentation was investigated. In cultures of Caldicellulosiruptor saccharolyticus grown on sucrose and Thermotoga elfii grown on glucose stoichiometries of 3.3 mol of hydrogen and 2 mol of acetate per mol C6-sugar unit were obtained. The hydrogen level was about 83 f the theoretical maximum. C. saccharolyticus and T. elfii reached maximum cell densities of 1.1x109 and 0.8x109 cells/ml, respectively, while their maximum hydrogen production rates were 11.7 and 5.1 mmol/g dry weight/h, respectively. For growth of C. saccharolyticus on sucrose, a biomass yield of 45.1 gDW/mol sucrose and a YATP of 11.3–14.1 were calculated. Replacement of yeast extract by casamino acids, plus proline and vitamins in the medium of C. saccharolyticus resulted in similar yields of hydrogen production on sucrose, but diminished the rate by about 30&Eth;Both yeast extract and tryptone were required by T. elfii, and appeared to function as sources of carbon, nitrogen and energy. In the absence of tryptone, T. elfii converted 26 f the glucose to another by-product, resulting in a lower yield of hydrogen. Growth of T. elfii ceased prior to glucose depletion, but the culture continued to ferment glucose to hydrogen and acetate until all glucose was consumed.