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.