Strain K11T was isolated from activated sludge of a municipal wastewater-treatment plant. Phylogenetic analysis of the 16S rRNA gene sequence revealed that it represents a distinct line of descent within the Comamonadaceae. The novel strain was a Gram-negative, catalase- and oxidase-positive, non-motile, straight to slightly curved rod. Polyhydroxyalkanoate granules were stored intracellularly as reserve material. Colonies on agar plates were small, regular and characterized by a water-insoluble yellow pigment. Unbranched fatty acids 16:1omega7c, 16:0 and 18:1omega7c dominated the cellular fatty acid pattern and ubiquinone-8 (Q-8) was the major component of the respiratory lipoquinones, both traits typical of members of the Comamonadaceae. A distinguishing characteristic was the presence of the two hydroxy fatty acids 10:0 3-OH and 12:0 2-OH, each in significant amounts. The G+C content of the DNA was 59 mol%. Strain K11T was capable of aerobic chemolithoheterotrophic growth using thiosulfate as an additional substrate, but could not grow autotrophically with thiosulfate or hydrogen. Facultative anaerobic growth was possible with nitrate and nitrite as electron acceptors, but not with ferric iron, sulfate or by fermentation. The sole end product of denitrification was N2O; nitrite accumulated only transiently in small amounts. Based upon phylogenetic and phenotypic evidence, it is proposed to establish the novel taxon Ottowia thiooxydans gen. nov., sp. nov., represented by the type strain K11T (=DSM 14619T=JCM 11629T). Aquaspirillum gracile was among the phylogenetically most closely related species to strain K11T. This species has been wrongly classified, and it is proposed to reclassify it as Hylemonella gracilis gen. nov., comb. nov. The type strain is ATCC 19624T (=DSM 9158T).