A new thermophilic, carbohydrate-fermenting, obligately anaerobic bacterial species was isolated from a runoff channel formed from flowing bore water from the geothermally heated aquifer of the Great Artesian Basin of Australia. The cells of this organism were nonsporulating, motile, gram negative, and rod shaped and generally occurred singly or in pairs. The optimum temperature for growth was 65 to 68 degrees C, and no growth occurred at temperatures below 44 degrees C or above 80 degrees C. Growth was inhibited by 10 micrograms of lysozyme per ml, 10 micrograms of penicillin per ml, 10 micrograms of tetracycline per ml, 10 micrograms of phosphomycin per ml, 10 micrograms of vancomycin per ml, 10 micrograms of vancomycin per ml, and NaCl concentrations greater than 0.2%. The optimum pH for growth was 7.0, and no growth occurred at pH 5.5 or 8.5. The DNA base composition was 35 mol% guanine plus cytosine, as determined by thermal denaturation. The end products of glucose fermentation were lactate, acetate, ethanol, CO2, and H2. Sulfur, but not thiosulfate, sulfite, or sulfate, was reduced to sulfide. Phase-contrast microscopy of whole cells and an electron microscopic examination of thin sections of cells revealed the presence of single terminal spheroids, a trait common in members of the genus Fervidobacterium. However, a phylogenetic analysis of the 16S rRNA sequence revealed that the new organism could not be assigned to either of the two previously described Fervidobacterium species. On the basis of these observations, we propose that the new organism should be designated a new Fervido-bacterium species, Fervidobacterium gondwanense. The type strain of this species is strain AB39 (= Australian Collection of Microorganisms strain ACM 5017).