A continuous stirred tank reactor with and without sulfur recovery has been operated using Chlorobium thiosulfatophilum for the conversion of H[sub 2]S to elemental sulfur. In operating the reactor system with sulfur recovery, a gas retention time of 40 min was required to obtain a 100 percent conversion of H[sub 2]S to elemental sulfur. Essentially no SO[sub 4][sup 2[minus]], an undesirable product, was produced under these conditions. Significant reductions in the gas retention time are expected by employing cell recycle after sulfur recovery, and by using increased pressure.