A nitrate-reducing bacterium, strain SPR2(T), was isolated from a sulphur-oxidizing, autotrophic denitrification reactor used for advanced treatment of wastewater from the lake of Sihwa, Korea. The strain was aerobic but could grow under anaerobic conditions. It was Gram-positive, exhibited rough white colonies on complex nutrient agar, produced elementary branching hyphae that fragmented into rod/coccus-like elements and showed chemotaxonomic markers that were consistent with classification in the genus Gordonia, i.e. meso-diaminopimelic acid, arabinose and galactose in whole-cell hydrolysates, N-glycolylmuramic acid in the peptidoglycan wall, unbranched saturated and monounsaturated fatty acids plus tuberculostearic acid (TBSA), mycolic acids that comprised 48-56 carbon atoms and MK-9(H(2)) as the predominant menaquinone. The 16S rDNA sequence of strain SPR2(T) showed highest similarity to Gordonia amicalis DSM 44461(T) and Gordonia hydrophobica DSM 44015(T), with values of 98.2 and 97.9 %, respectively. These values were far below 99.5 % (usually found at the intraspecies level) and they were in the range that separates species at the intrageneric level. The separate phylogenetic position of SPR2(T) was supported by differences in fatty acid and mycolic acid compositions and in carbon utilization tests that distinguished strain SPR2(T) from all known Gordonia species. Combined genotypic and phenotypic data show that strain SPR2(T) merits recognition as a novel species within the genus Gordonia, for which the name Gordonia sihwensis sp. nov. is proposed; the type strain is SPR2(T) (=DSM 44576(T)=NRRL B-24155(T)).