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.