We have tested the hypothesis (Van Ooteghem et al. Appl Biochem Biotechnol 2002 98-100: 177-189) that microaerobic metabolism may increase the yield of H(2) from the thermophilic bacterium Thermotoga neapolitana. In anaerobic conditions, T. neapolitana converted glucose into acetic acid and lactic acid and yielded 2.4 +/- 0.3 mol H(2) mol(-1) glucose. The bacterium tolerated low O(2) partial pressures but the H(2) yield was not improved under microaerobic conditions. Our results indicate that T. neapolitana only produces H(2) by anaerobic metabolism, and that the yield of H(2) can be maximised by minimising the production of lactic acid.