Manganese-containing superoxide dismutase was isolated from an extreme thermophile, Thermus thermophilus HB8. About 150 mg of the enzyme was obtained from 500 g of wet cells. The enzyme was easily crystallized in octahedra from ammonium sulfate solution. The molecular weight of the enzyme was determined to be 8.2 X 10(4) and 8.4 X 10(4) by sedimentation equilibrium and gel-filtration, respectively. The enzyme contains 2 atoms of manganese per mole and consists of four subunits of identical molecular weight, about 2.1 X 10(4). The amino acid composition of the enzyme is similar to that of the superoxide dismutase of Thermus aquaticus. Proline was detected as the N-terminal amino acid. The isoelectric point was determined to be pH 6.0 by the electrofocusing method. The enzyme has maxima at 283 nm and 480 nm in the absorption spectrum. The CD spectrum suggests that the enzyme has a high alpha-helical content.