A novel marine hyperthermophile, Thermococcus onnurineus NA1, was found to grow on Cl carbon compounds, such as formate and carbon monoxide (CO), and produce hydrogen (H-2). In the present study, the growth and H-2 production of NA1 were examined to determine its potential as H-2 producer. NA1 showed relatively high specific growth rates, 0.48 h(-1) and 0.40 h(-1) with CO (20%, v/v) and formate (100 mM), respectively, when cultivated in batch mode in a minimal salt medium fortified with 1.0 g L-1 yeast extract. On the other hand, cell growth in both cases stopped at approximately 6 h and the final cell densities were extremely low at 18.2 and 12.1 mg protein L-1 with CO and formate, respectively. The maximum final cell density could be improved greatly to 36.0 mg protein L-1 by optimizing CO content (50%, v/v) and yeast extract concentration (4.0 g L-1), but it was still very low. During the cell growth, formate and CO were used as energy source rather than carbon source. In the resting cell experiments, NA1 exhibited remarkably high H-2 production activities as 385.0 and 207.5 mu mol mg protein(-1) h(-1) for CO and formate, respectively. When formate (100 mM) or CO (100%, v/v) was added repeatedly at 30-35 h intervals, NA1 showed consistent H-2 production for 3 cycles with a yield of approximately 1.0 mol H-2 mol(-1) for both CO and formate. This study suggests that T. onnurineus NA1 has a high H-2 production potential from formate or CO but a method for achieving a high cell density culture is needed. Copyright (c) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.