An anaerobic thermophilic bacterium designated CA9F1 was isolated from a thermal spring in France. Strain CA9F1 was observed to grow at temperatures between 55 and 70 °C (optimum 65 °C) and at pH between 6.8 and 9.5 (optimum pH 7.4). Strain CA9F1 does not require salt for growth (0-10 g l(-1) NaCl), with an optimum at 1 g l(-1). The DNA G+C content was determined to be 38.5 mol% (Tm). The major cellular fatty acids identified were C15:0, C16:0, C17:0 iso. Based on phenotypic, chemotaxonomic and genotypic properties, strain CA9F1 was identified as Thermovenabulum gondwanense and this species was studied in more detail. Strain CA9F1 is a Gram-positive bacterium which forms a complex and regular multilayered cell wall structure, here characterised as being due to the presence of an S-layer. The network covers the entire cell surface and forms a hexagonal structure resembling that observed for Deinococcus radiodurans. The main protein component of the S-layer possesses domains comparable to that of the S-layer protein of Halothermothrix orenii. The characteristics of the strain were compared to that of T. gondwanese R270(T) isolated from microbial mats thriving in the thermal waters of a Great Artesian Basin bore runoff channel at 66 °C, in Australia. Significant differences were observed between CA9F1 and the type strain. One of the major physiological differences is the inability of CA9F1 to reduce Fe(III). An emended description of T. gondwanense is given.