New alkaliphilic anaerobic fermentative bacteria, strains Z-7981 and Z-7981, with Gram-positive cell walls, were isolated from the cellulolytic community from the soda lake Nizhnee Beloye, south-east of Baikal. Cells were motile rods, which differed in dimensions but, according to 98% DNA/DNA homology, belonged to the same species. Strain Z-7981 was chosen as the type and studied in detail. It did not produce spores and its cells were non-thermoresistant. It was a true alkaliphile with a growth range from pH7.1 to pH10.1 and optimal pH for growth at pH9.1. It was obligately dependent on Na+ and carbonate ions but not on Cl. Growth occurred in media with total sodium content from 0.076M to 1.27M Na+ with a broad optimum from 0.25 to 0.86M Na+. Growth showed an optimum at 35C, with absence of growth above 46C. The organism was aerotolerant and was capable of fermentation in non-reducing medium at less than 4.75% O2 in the gas phase. Strain Z-7981 fermented mono- and disaccharides, sugar alcohols, but only glutamate and cysteine among the amino acids, and the proteinaceous substrates, chitin and dried Spirulina biomass. Fermentation products were acetate and ethanol. Fe3+ was reduced in a process that yielded no energy. Phylogenetically the new organism belonged to cluster XI of the Gram-positive bacteria with low G+C content and its closest neighboring taxon was Tindallia magadiensis. However, according to its phenotypic and genotypic characters it did not belong to any known genus from this group. We suggest a new genus and species with the name Anoxynatronum sibiricum and strain Z-7981 as its type (=DSM15060).