A novel, obligately anaerobic, alkalithermophilic, chemo-organotrophic bacterium was isolated from the sediment of an alkaline hot spring located on Paoha Island in Mono Lake, California, USA. This rod-shaped bacterium was motile via peritrichous flagella. Isolated strains grew optimally in 5-25 g NaCl l(-1), at pH 9.0-9.5 and at a temperature of 58 degrees C and were fermentative and mainly proteolytic, utilizing peptone, Casamino acids and yeast extract. Optimal growth was seen in the presence of elemental sulfur, polysulfide or thiosulfate with concomitant reduction to hydrogen sulfide. Sulfite was also formed in an equal ratio to sulfide during reduction of thiosulfate. The novel isolate could also reduce Fe(III) and Se(IV) in the presence of organic matter. On the basis of physiological properties, 16S rRNA gene sequence and DNA-DNA hybridization data, strain PAOHA-1(T) (=DSM 14826(T)=UNIQEM 227(T)) belongs to the genus Anaerobranca and represents a novel species, Anaerobranca californiensis sp. nov.