Abstract: A biological process is described involving the conversion of SO.sub.2 to H.sub.2 S using sulfate reducing bacteria in co-culture with facultatively anaerobic heterotrophs under anaerobic conditions in a maintenance medium which includes a substrate fermentable by the facultatively anaerobic heterotrophs to produce an end product which acts as the carbon and energy source for the sulfate reducing bacteria. Glucose and sewage sludge are the two fermentable substrates described. The process is described as being used in conjunction with a process for removing SO.sub.2 from a flue gas and then converting the SO.sub.2 to H.sub.2 S. The H.sub.2 S may be reacted with some remaining SO.sub.2 in a Claus reactor to produce elemental sulfur or the H.sub.2 S may be used as a reducing gas to regenerate the sorbent being used to remove the SO.sub.2 from the flue gas.