The cells of Thiobacillus thiooxidans, which had been in contact with sulfur or sulfide in air (or CO2-free air), could fix added COa very rapidly after replacing air with nitrogen. This fixation is designated as the postoxidative fixation. “Preoxidation” of the sulfur compounds is mandatory for the occurrence of the postoxidative fixation. The cells which had preliminarily oxidized sulfide could not show the CO2-fixation, when they were placed under an anaerobic condition in the absence of the sulfur compound. These results indicate that sulfur compounds may have an important role as the electron donor for the reduction of CO2, besides their role as the substrate of respiration to secure energy for the fixation of CO2