The strains NAV-I, NAV-2, NAV-3 of new nonsporing sulfate-reducing bacteria with spherical to oval motile cells were isolated with nicotinic acid as electron donor and carbon source. All strains were obtained from marine sediment samples. Growth occurred in defined anaerobic salt water media supplemented with biotin and thiamine as growth factors. Utilization of nicotinic acid depended on the addition of selenite (10-8-10 -7 tool/l); requirement for molybdate was not detected. Further compounds utilized as electron donors and carbon sources were hydrogen plus carbon dioxide, formate, acetate, propionate, higher fatty acids up to 16 carbon atoms, alcohols, pyruvate, malate, fumarate, succinate, glutarate, glutamate and pimelate. On hydrogen plus carbon dioxide or on formate, slow growth was obtained without an additional organic carbon source. Growth on acetate or propionate as sole organic substrates was possible, however, it was extremely slow. Stoichiometric measurements revealed that nicotinic acid was completely oxidized to carbon dioxide and ammonia. The average growth yield was 38 g cell dry weight per tool of nicotinic acid used. 3-Phenylpropionate was oxidized to carbon dioxide and benzoate; benzoate could not be oxidized further. The strains were able to reduce sulfite or thiosulfate instead of sulfate. No growth on organic compounds was observed in the absence of an electron acceptor. In the cell membrane fraction, b-type cytochrome was identified; desulfoviridin was not detected. Cell-free extracts oxidized nicotinic acid, nicotinamide or pyrazine-2-carboxylic acid with methylviologen as electron acceptor. Strain NAV-1 is described as type strain of the new species Desutfococcus niacini.