A novel, extremely halophilic, neutrophilic archaeon was isolated from a mixed sediment sample from different hypersaline lakes in Kulunda steppe (Altai, Russia) at 4 M NaCl with acetate and thiosulfate as substrates. The enrichment culture developed in two phases. During the first phase, a rapid growth of heterotrophic, red-colored, polymorphic rods occurred with the concomitant oxidation of thiosulfate to tetrathionate. The latter was subsequently oxidized to sulfate during a second, slower phase by extremely halophilic, chemolithoautotrophic bacteria belonging to the gamma subdivision of the Proteobacteria. The archaeal strain HG 1 was isolated from the first phase of the enrichment culture using acetate as substrate. It was able to oxidize thiosulfate to tetrathionate during heterotrophic growth with acetate-a property not yet demonstrated for any of the known haloarchaea. The presence of tetrathionate synthase, the enzyme responsible for thiosulfate oxidation, was detected in strain HG 1. The activity was associated with membranes and depended specifically on Cl-, in contrast to the similar activity in extremely halophilic sulfur-oxidizing Gammaproteobacteria from the same enrichment, which was soluble and demanded both Na+ and Cl- . Strain HG 1 was identified as a member of the genus Natronorubrum.