Continuous hydrogen production from glucose at 75 degrees C was examined at HRTs of 0.67, 1, 2, and 3 d by using extreme thermophilic microflora, which consisted of several subspecies of Caldanaerobacter subterraneus. The maximum hydrogen yield of 3.32 mol-H-2/mol-glucose degraded was obtained at the highest HRT, 3 d. Both the maximum volumetric and specific rates of hydrogen production (1.22 L-H-2/L/d and 4.15 L-H-2/g-VSS/d, respectively) were obtained at the lowest HRT, 0.67 d. A 16S rDNA analysis showed that the community structure of the microflora did not change at any of the HRTs. (C) 2008, The Society for Biotechnology, Japan. All rights reserved.