A S-oxidizing enzyme was purified to homogeneity from cell-free exts. of mixotrophically grown cells of S. brierleyi. The enzyme catalyzed the oxidn. of elemental S to sulfite and had a mol. wt. of 560,000. Only 1 type of subunit (mol. wt. = 35,000) could be detected. The pH and temp. optima for activity were 7.0 and 65 Deg, resp., and an apparent Km for S was calcd. to be 0.05M. Enzyme assays with 18O-enriched O2 demonstrated directly that atm. O was attached to S. The 1st detectable reaction product was sulfite. Thus, S oxidn. on S. brierleyi is performed by an enzyme of the oxygenase type.