The hyperthermophilic archaeon, Thermococcus onnurineus, was grown in media supplemented with either CO, formate, or starch. H-2 was produced with each substrate with respective maximum rates of 1.55, 3.83 and 2.66 mmol H-2/l h. The yields (mol H-2/mol substrate) were 0.98, 1 and 3.13, respectively. This microbe is the first example where a single microorganism can grow and produce H-2 using CO, formate or starch as substrate.