Growth physiology of the iron-reducing bacteria Thermoterrabacterium ferrireducens and Thermoanaerobacter siderophilus was investigated. The stimulation of the organotrophic growth of T. ferrireducens and T. siderophilus in the presence of Fe(III) was shown to be due to the utilization of ferric iron as an electron acceptor in catabolic processes and not to the effect exerted on the metabolism by Fe(II) or by changes in the redox potential. It was established that Fe(III) reduction in T. ferrireducens is not a detoxication strategy. In T. siderophilus, this process is carried out to relieve the inihibitory effect of hydrogen. T. ferrireducens was shown to be capable of lithoautotrophic growth with molecular hydrogen as electron donor and amorphous ferric oxide as electron acceptor, in the absence of any organic substances. The minimum threshold of H2 consumption was 3 x 10(-5) vol % of H2. The presence of CO dehydrogenase activity in T. ferrireducens suggests that CO2 fixation in this organism involves the anaerobic acetyl-CoA pathway. T. siderophilus failed to grow under lithoautotrophic conditions. The fact that T. ferrireducens contains c-type cytochromes and T. sidrophilus lacks them confirms the operation of different mechanisms of ferric iron reduction in these species.