Thiobacillus denitrificans is a chemoautotroph and a facultatively anaerobic bacterium that can use reduced sulfur compounds as energy sources with oxidation to sulfate. Under anoxic conditions, nitrate may serve as a terminal electron acceptor with reduction to elemental nitrogen. T. denitrificans has previously been shown to remove hydrogen sulfide from sour gases under both aerobic and anoxic conditions. Hydrogen sulfide concentrations were reduced from 10 000 ppmv to undetectable levels with 1-2 s of gas-liquid contact time in bench-scale fermenters. The oxidation product, sulfate, accumulated in the liquid phase of the culture medium. Flocculated T. denitrificans has now been used to remove hydrogen sulfide (up to 1500 ppmv) from a sour gas in a 0.5-m(3) pilot-scale bubble column. The bubble column was operated for 7 weeks under aerobic conditions, with gas feeds of air and hydrogen sulfide in nitrogen. Up to 97% removal of hydrogen sulfide was observed with complete oxidation to sulfate, which accumulated in the reactor medium. Limitations in H2S removal were due to mass transfer and not the microbiology of the system. More complete H2S removal can be accomplished by increasing gas-liquid contact in a single reactor or using two bubble columns in series.