Three different species of the genus Sulfolobus, S. acidocaldarius, S. solfataricus (= Caldariella) and S. brierleyi, have been distinguished by the conditions required for optimal growth, by the component patterns of their DNA-dependent RNA polymerases and by DNA sequence data. Many isolates of these species are able to grow chemolithoautotrophically using CO2 as the sole carbon source and the oxidation of S(0) with O2 yielding sulphuric acid, as the energy source, though a few others grow only heterotrophically. We show here that a strain of a novel Sulfolobus species, S. ambivalens, is alternatively able to live by an anaerobic mode of chemolithoautotrophy, also using CO2 as the sole carbon source, but using reduction of S(0) with H2, yielding H2S as the energy source. This mode of growth is correlated with the amplification of a plasmid, pSL10.