The taxonomic positions of 'Pseudomonas hydrogenothermophila' strain TH-1 and 'Flavobacterium autothermophilum' strain TH-4 were studied by 16S rDNA sequencing. These organisms are Gram-negative, strictly aerobic, thermophilic, facultatively chemolithoautotrophic hydrogen-oxidizing rods and have a DNA G + C content of 63-65 mol%. The major isoprenoid quinone is ubiquinone-8 and 3-hydroxy decanoic acid (3-OH C10:0) is the major 3-hydroxy cellular fatty acid. A phylogenetic analysis based on 16S rDNA sequences placed strains TH-1T and TH-4 in the beta-subclass of the Proteobacteria. The taxonomic characteristics of these organisms are different from those of previously described aerobic, facultatively chemolithoautotrophic, hydrogen-oxidizing bacteria that belong to the beta-subclass of Proteobacteria. On the basis of the information described above, a new genus and species, Hydrogenophilus thermoluteolus gen. nov., sp. nov., is described to include both strains. The type strain is strain TH-1T (= IFO 14978T).