Growth of Thiobacillus thiooxidans 8085 on 9K medium containing thiosulfate instead of sulfur, as the sole energy source, was successfully attained. The optimum growth pH was maintained at 5.0 during cultivation. The cultivation period for T. thiooxidans on thiosulfate was 4-5 days at 28$\sp{\circ}$C. The final cell yield with 1% thiosulfate in a 4 liter reactor was 425 mg wet cells/L. The culture was continuously maintained with fresh medium. The harvested cells were washed and stored in 0.1M sodium citrate pH 6.0 buffer at 4$\sp{\circ}$C where full activity of the cells remained after 3 weeks. The mechanism of thiosulfate oxidation in T. thiooxidans 8085 was studied at pH 5.0 as well as pH 2.3 and pH 7.0. The oxidation of other reduced inorganic sulfur substrates tetrathionate, sulfur dissolved in dimethyl sulfoxide (DMSO), sulfur suspended in Tween 80 and sulfite were also studied. The cytochrome ystem of T. thiooxidans 8085 in both whole cells and cell-free crude extracts was also studied with various substrates, inhibitors and uncouplers. (Abstract shortened by UMI.)