Seven strains of extremely halophilic and obligately chemolithoautotrophic sulfur-oxidizing bacteria (SOB) were enriched and isolated at 4 M NaCl from sediments of hypersaline inland lakes in south-eastern Siberia and a Mediterranean sea solar saltern. Cells of the novel isolates were spindle-like, long and non-motile rods with a Gram-negative type of cell wall. They were obligately chemolithoautotrophic SOB using thiosulfate and tetrathionate as electron donors and represent the first example of extremely halophilic chemolithoautotrophs that are able to grow anaerobically with nitrate as electron acceptor. The characteristic feature of the group was the production of large amounts of tetrathionate as an intermediate during the oxidation of thiosulfate to sulfate. With thiosulfate, the novel strains grew within the pH range from 6.5 to 8.2 (optimum at pH 7.5-7.8) and at NaCl concentrations from 1.5 to 4.0 M (optimum at 3.0 M). Cells grown at 4 M NaCl accumulated extremely high concentrations of glycine betaine as a compatible solute. The dominant cellular fatty acids were 10MeC(16 : 0) and C(16 : 0). Based on the DNA-DNA relatedness values, the isolates consisted of a single genomic species and had a similar phenotype. Phylogenetic analysis placed the novel bacteria in the class Gammaproteobacteria as an independent lineage with no significant relationship to any other genera in this class. On the basis of phenotypic and genotypic analysis, the group is proposed to represent a new genus, Thiohalorhabdus gen. nov., with Thiohalorhabdus denitrificans gen. nov., sp. nov. as the type species (type strain HL 19(T)=DSM 15699(T)=UNIQEM U223(T)).