This study was conducted with the purpose of evaluating the possibilities of biologically oxidizing thiosulfate in a fluidized bed. A laboratory scale, autotrophic fluidized bed was continuously operated using a synthetic thiosulfate waste. The reactor was maintained at a hydraulic loading rate of 16.1 gpm/ft² and an influent thiosulfate concentration of 100 mg/1. The void space detention time at this loading rate was approximately 0.6 minutes. Through a series of influent pH adjustments, the fluidized bed demonstrated the ability to remove up to 75 percent of the applied thiosulfate, but the intermediate thiosulfate metabolic product, polythionate, proved difficult to oxidize. Due to the high oxygen demand of thiosulfate and the high concentration of volatile solids in the reactor, aeration with high purity oxygen was required to maintain an aerobic environment. Continuous operation of the fluidized bed also verified the preference of Thiobacillus bacteria to attach to a solid surface. Ammonia nitrogen and phosphorus requirements of the fluidized bed Thiobacillus culture were evaluated in a series of batch and oxygen uptake experiments. Unlike nitrogen, phosphorus was demonstrated to be an influential nutrient in the oxidation of thiosulfate. Trace nutrients which included zinc, iron, molybdenum, copper, and cobalt were also shown to influence the metabolic activity of the Thiobacilli. Acid decomposition of thiosulfate was briefly evaluated. Acidification below a pH of 3.0 resulted in rapid thiosulfate destruction and suggested a second mechanism for the removal of thiosulfate in biological waste treatment systems. / Doctor of Philosophy