A novel obligately anaerobic, mesophilic, alkaliphilic spirochaete, strain ASpC2T, was isolated from an anaerobic sediment of alkaline, hypersaline Owens Lake in California, USA. The Gram-negative cells are motile, helical in shape and 0.23×8.0–18.0 μm. Growth occurs within the following ranges: 13–41 °C, with optimal growth at 35 °C; 1–3 % (w/v) NaCl, with optimal growth at 2 % (w/v) NaCl; and pH 7.8–10.5, with optimal growth at pH 10.0. The novel isolate is strictly alkaliphilic and requires high concentrations of carbonate ions in the medium. It utilizes some sugars, some organic acids, some amino acids, Casamino acids, yeast extract and peptone. The main end products of glucose fermentation are CO2 and acetate. Strain ASpC2T is resistant to kanamycin and rifampicin, but sensitive to ampicillin, chloramphenicol, gentamicin and tetracycline. The DNA G+C content of the new isolate is 43.8 mol%, its genome size is 6×108 Da and the melting temperature of its genomic DNA is 71 °C. DNA–DNA hybridization experiments demonstrated 46 % similarity with the phylogenetically most closely related species, Spirochaeta asiatica Z-7591T. On the basis of physiological and molecular properties, the new isolate belongs taxonomically to a novel species within the genus Spirochaeta, for which the name Spirochaeta dissipatitropha sp. nov. is proposed (type strain, ASpC2T=ATCC BAA-1083T=JCM 12856T). S. dissipatitropha ASpC2T is the second strain in the genus (after Spirochaeta smaragdinae SEBR 4228T) that is able to use proteolysis products as the sole energy source, and additional tests have shown that other halo-alkaliphilic spirochaetes (Spirochaeta americana, Spirochaeta alkalica and Spirochaeta africana) are also able to grow on yeast extract alone; therefore, an emended description for the genus Spirochaeta is given.