The homoacetogenic bacteria Acetobacterium woodii, A. carbinolicum, Sporomusa ovata, and Eubacterium limosum, the methanogenic archaeon Methanobacterium formicicum, and the sulfate-reducing bacterium Desulfotomaculum orientis all produced formate as an intermediate when they were growing: chemolithoautotrophically with H-2 and CO2 as sources of energy, electrons, and carbon. The sulfate-reducing bacterium Desulfovibrio vulgaris grew chemolithoheterotrophically with H-2 and CO2 using acetate as carbon source, but also produced formate when growth was limited by sulfate. All these bacteria were also able to grow on formate as energy source. Formate accumulated transiently while H-2 was consumed. The maximum formate concentrations measured in cultures of A. woodii and A. carbinolicum were proportional to the initial H-2 partial pressure, giving a ratio of about 0.5 mM formate per 10 kPa H-2. The methanogen Methanobacterium bryantii, on the other hand, was unable to grow on formate and did not produce formate during chemolithoautotrophic growth on HZ The results indicate that the ability to utilize formate, that is, to possess a formate dehydrogenase, was the precondition for the production of formate during chemolithotrophic growth on Hz.