Several Thermotogales, previously reported as being strict anaerobes, have demonstrated the ability to grow and produce hydrogen in the presence of moderate amounts of molecular oxygen. Thermotoga neapolitana seems to be less sensitive to O-2 than other members of this order, including Thermotoga maritima, whose hydrogenase has been purified and characterized. Instead, the enzyme responsible for the hydrogen production by T. neapolitana has not yet been identified. After the recent sequencing of the T. neapolitana genome, it has been possible to search for the orthologous gene responsible for this unusal hydrogenase activity. By means of in silico analysis, we built a molecular model for both T. maritima and T neapolitana proteins and analyzed conservation, focusing on the subtle structural differences responsible for the increased oxygen resistance in the latter and underscoring two mutations (E475S and T539L) which represent a specific adaption for more effective release of hydrogen in aerobic conditions. (C) 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.