~ During studies on a desulphurizing, denitrifying effluent-treatment system, an organism which is able to grow aerobically and anaerobically on reduced sulphur compounds and hydrogen, while fixing carbon dioxide, was isolated. The new isolate is also capable of mixotrophic and heterotrophic growth on a wide range of substrates, and is therefore a facultatively aerobic, facultative autotroph. Comparisons with two similar species, Thiobacillus A2 and Paracoccus denitrijicans, showed that the new isolate is significantly different from the other two, and merits separate classification. In view of its ability to oxidize reduced sulphur compounds, and because it is a chain-forming coccus rather than a rod, the new isolate has been given the generic name of Thiosphaera, and the species name pantotropha in recognition of its wide range of possible substrates.