Methanococcus thermolithotrophicus ST22 was isolated from produced water of a North Sea oil field, on mineral medium with H-2-CO2 as the sole source of carbon and energy. The isolate grew at 17 to 62 degrees C, with an optimum at 60 degrees C. The pH range was 4.9 to 9.8, with optimal growth at pH 5.1 to 5.9; these characteristics reflected its habitat. Strain ST22 was quickly identified and distinguished from the type strain by immuno-blotting.