~~~ A thermophilic, facultatively chemolithoautotrophic bacterium oxidizing molecular hydro- gen has been isolated. It formed round, terminal endospores and had a multilayered cell wall. Although growth rates were highest under autotrophic conditions, certain organic chemicals, including short-chain fatty acids, some amino acids, 1-propanol and phenol, could serve as sole carbon and energy sources; ammonium ions, urea and L-asparagine could serve as sole nitrogen sources. The hydrogenase was membrane-bound and did not reduce NAD(P). The guanine+cytosine content of its DNA was 67 to 68 molyo. Based on its morphological and biochemical features, the organism belongs to Bacillus, but differs from any of the previously described species attributed to this genus. It is therefore proposed as constituting a new species, Bacillus schlegelii, named after the German micro- biologist H. G. Schlegel. The type strain of B. schlegelii, strain MA48, has been deposited with the German Collection of Microorganisms, Gottingen, under the accession number DSM 2000.