A biosensor based onThiobacillus thioparuswas developed for the determination of thiosulfate and methanethiol.Thiobacillus thioparusis a chemoautotrophic bacterium and it oxidizes sulfur compounds to sulfuric acid. The sensor consisted of an oxygen electrode and immobilizedT.thioparus. When the sensor was used to determine thiosulfate, a linear relation between sensor output and concentration was obtained for the concentration range from 1 to 100 μM in a batch system and from 1 to 10 mM in a flow injection system. Output of the sensor was stable for more than 1 month. For methanethiol, the response of the sensor was measured for the concentration range from 0.2 to 3 mM in a flow injection system.Key words: microbial sensor, thiosulfate, methanethiol,Thiobacillus thioparusTK-m.