The genome of the sulfate-reducing bacterium Desulfovibrio vulgaris Hildenborough encodes three formate dehydrogenases (FDHs), two of which are soluble periplasmic enzymes (FdhAB and FdhABC(3)) and one that is periplasmic but membrane-associated (FdhM). FdhAB and FdhABC(3) were recently shown to be the main enzymes present during growth with lactate, formate or hydrogen. To address the role of these two enzymes, Delta fdhAB and Delta fdhABC(3), mutants were generated and studied. Different phenotypes were observed in the presence of either molybdenum or tungsten, since both enzymes were important for growth on formate in the presence of Mo, whereas in the presence of W only FdhAB played a role. Both Delta fdhAB and Delta fdhABC(3) mutants displayed defects in growth with lactate and sulfate providing the first direct evidence for the involvement of formate cycling under these conditions. In support of this mechanism, incubation of concentrated cell suspensions of the mutant strains with lactate and limiting sulfate also gave elevated formate concentrations, as compared to the wild-type strain. In contrast, both mutants grew similarly to the wild-type with H-2 and sulfate. In the absence of sulfate, the wild-type D. vulgaris cells produced formate when supplied with H-2 and CO2, which resulted from CO2 reduction by the periplasmic FDHs. The conversion of H-2 and CO2 to formate allows the reversible storage of reducing power in a much more soluble molecule. Furthermore, we propose this may be an expression of the ability of some sulfate-reducing bacteria to grow by hydrogen oxidation, in syntrophy with organisms that consume formate, but are less efficient in H-2 utilization.