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Forty-three strains of obligately chemolithoautotrophic sulfur-oxidizing bacteria were isolated from highly alkaline soda lakes in south-east Siberia (Russia) and in Kenya using a specific enrichment procedure at pH 10. The main difference between the novel isolates and known sulfur bacteria was their potential to grow and oxidize sulfur compounds at pH 10 and higher. The isolates fell into two groups that were substantially different from each other physiologically and genetically. Most of the Siberian isolates belonged to the group with a low DNA G+C content (48.0-51.2 mol%). They were characterized by a high growth rate, a low growth yield, a high cytochrome content, and high rates of oxidation of sulfide and thiosulfate. This group included 18 isolates with a DNA homology of more than 40%, and it is described here as a new genus, Thioalkalimicrobium, with two species Thioalkalimicrobium aerophilum (type …