Tlie influence of various 0,-concentrations on chemoautotrophic growth was dcterniincd in thc range from microaerobic t o aerobic conditions. Chromatium vinosurn D, Thiocystis violacea 231 1 and Chromatiurn yracile 25-1 were grown chemoautotropliically using consecutively repeated batch riiltures. Rapid growth of all thrce strains occurred only under microaerobic conditions. Thc fastcst growth (1.c = 0.058) was obtained with Chromatium gracile 25-1 grown in the prcsence of 1 mg 0, . 1 -l. For optimal chemoautotrophic growth aulfur-containing cells had to bc supplied continuously with cxtracellular electron donor. Multiplication of Thiocystis violacea 231 1 and Chromatiwn gracile 25-1 was significantly inhibited after several cell generations, while Chromatiurn winosum D could be grown unlimited. I n case of the latter bacterium any given 0,-concentration determined il unique growth rate and a unique specific bclil a content. The analysis of absorption spect.ra of whole cclls revealed that different pigment complexes were regulated separately by oxygen. Growth yields were not dependent on the 0,-concentration. A growth yield of 12 g dry cell mass per mol of thiosulfate oxidized was determined for Chromatium vinosum D growing in the micro-to semiaerobic 0,-concentration range. When compared to phototrophically grown cells, the catalase content of chemotrophically grown cells was 2 times lower while the maximal specific respiration rate of the cells was 7 times higher. The ecological significance of the kinetics of chemotrophic growth under various growth conditions is discussed.