Biohydrogen fermentation by the hyperthermophile Thermotoga neapolitana was conducted in a continuously stirred anaerobic bioreactor (CSABR). The production level of H-2 from fermentation in a batch culture with pH control was much higher than without pH control from pentose (xylose) and hexose (glucose and sucrose) substrates. The respective H-2 yield in the batch culture with pH control from xylose and glucose was 2.22 +/- 0.11 mol-H-2 mol(-1) xylose(consumed) and 3.2 +/- 0.16 mol-H-2 mol(-1) glucose(consumed), which was nearly 1.2-fold greater for xylose and 1.6-fold greater for glucose than without pH control. In the case of sucrose, the H-2 yield from fermentation increased by 40.63%, compared with fermentation in batch cultures without pH control, from 3.52 +/- 0.171 to 4.95 +/- 0.25 mol-H-2 mol(-1) sucrose(consumed). The effects of stirring speed and different pH levels on growth and H-2 production were studied in the CSABR for highly efficient H-2 production. Growth and H-2 production of this bacterial strain in a batch culture with pH control or without pH control using a 3 L bioreactor was limited within 24 h due to substrate exhaustion and a decrease in the culture's pH. The pH-controlled fed-batch culture with a xylose substrate added in doses was studied for the prevention of substrate-associated growth inhibition by controlling the nutrient supply. The highest H-2 production rates were approximately 4.6, 4.1, 3.9, and 4.3 mmol-H-2 L-1 h(-1) at 32, 52, 67, and 86 h, respectively. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.