To study the process of microbial-mediated dolomite formation, growth experiments were carried out with selected bacterial cultures under anoxic environmental conditions simulating those found in Lagoa Vermelha, a hypersaline lagoon in Brazil where dolomite precipitation occurs. Specifically, we report the isolation of a particular strain of sulfate-reducing bacteria, LVform6, from Lagoa Vermelha sediment, which apparently promotes the formation of nonstoichiometric dolomite. Sulfate-reducing bacteria grown in a synthetic liquid medium produced dolomite during 30 days incubation at 30 degreesC. The precipitates have morphologies similar to those observed in Lagoa Vermelha sediment. Our results demonstrate that sulfate-reducing bacteria can influence dolomite precipitation under controlled low-temperature, anoxic conditions, and imply that anaerobic microorganisms can play an important role in carbonate sedimentation. They may have been particularly significant in Earth's earliest history when a more reducing atmosphere existed.