A denitrifying bacterium, designated strain FS T , was isolated from anoxic digested sludge on oestradiol [17 β -oestra-1,3,5(10)-triene-3,17-diol] or testosterone (17 β -hydroxyandrost-4-en-3-one) as the sole source of carbon and energy with nitrate as the electron acceptor. Strain FS T represents the first known bacterium to grow anaerobically on both oestradiol (C-18) and testosterone (C-19). Steroidal hormones were degraded completely by nitrate reduction to dinitrogen monoxide, which was further reduced to dinitrogen in stationary-phase cultures. Gram-negative cells were slightly curved rods, –×– μm in size, motile, non-fermentative, non-spore-forming and catalase- and oxidase-positive, showing optimal growth at pH , 28 °C and % (w/v) NaCl. Beside steroidal hormones, the bacterium utilized only a narrow range of organic substrates with nitrate as the electron acceptor, including several fatty acids and glutamate. No aerobic or anaerobic growth occurred on liquid or solid complex media. Phylogenetic analysis of the 16S rRNA gene sequence showed that strain FS T has no known close relatives and represents a distinct lineage within the Gammaproteobacteria . Together with the genera Nevskia , Hydrocarboniphaga , Solimonas and Sinobacter (less than 88 % 16S rRNA gene sequence similarity to strain FS T ), it forms a phylogenetic cluster separated from the families Chromatiaceae , Ectothiorhodospiraceae and Xanthomonadaceae . The quinone system of strain FS T consisted exclusively of ubiquinone Q-8. The dominant polar lipids were diphosphatidylglycerol and phosphatidylethanolamine. Spermidine in combination with putrescine and traces of sym -homospermidine were the basic polyamines. The major fatty acids detected in testosterone- or heptanoate-grown cells were C 15 : 0 and C 17 : 1 ω 8 c , minor hydroxylated fatty acids were C 11 : 0 3-OH and C 12 : 0 3-OH. The G+C content of the DNA was mol%. Based on the high 16S rRNA gene sequence divergence and different phenotypic properties from previously described gammaproteobacteria in combination with chemotaxonomic data, strain FS T is considered to represent a new genus and species, for which the name Steroidobacter denitrificans gen. nov., sp. nov. is proposed. The type strain of Steroidobacter denitrificans is FS T (=DSM 18526 T =JCM 14622 T ).