Growth characteristics of thermophilic hydrogen bacteria TH-1, and TH-4 were examined and taxonomic studies of these organisms were performed. Both Strains assimilated various organic acids but not the tested sugars. Autotrophic growth utilizing hydrogen, oxygen and carbon dioxide was more rapid than heterotrophic growth on organic acid media. With pyruvate as the sole carbon source, TH-1 grew more rapidly under oxy-hydrogen atmosphere (H2:02=4:1) than in air. Neither TH-1 nor TH-4 fit the description of any previously de-scribed species; and they are thus proposed as new species, Pseudomonas hydrogenothermophila nov. sp. Goto, Kodama and Minoda and Flavobacterium autothermophilum nov. sp. Goto, Kodama and Minoda, respectively.