The oxidation of elemental sulphur in Thiobacillus denitrificans is catalysed by an enzyme presentin the S 144 fraction ; GSH and a heat-stable factor are required for the reaction. Sulphur oxidation may be linked to oxygen uptake in the S 144 fraction and to nitrate reduction in the crude extract ( S }0). The initial product of sulphur oxidation is sulphite,