A novel nitrogen-fixing strain, designated DQS-4(T), was isolated from oil-contaminated soil in Taiwan and was characterized using a polyphasic taxonomic approach. Cells of strain DQS-4(T) stained Gram-negative, contained poly-�]-hydroxybutyrate granules and were motile rods, surrounded by a thin capsule. Cells displayed a strictly aerobic type of metabolism and fixed nitrogen microaerobically. Growth occurred at 10-45 �XC (optimum, 35-40 �XC), at pH 7.0-8.0 (optimum, pH 7.0) and with 0-2 % NaCl (optimum, 0.5-1 %). Phylogenetic analyses based on 16S rRNA gene sequences showed that strain DQS-4(T) belonged to the genus Azoarcus, and its closest neighbours were Azoarcus indigens VB32(T) and Azoarcus communis SWub3(T), with sequence similarities of 97.4 and 96.4 %, respectively. The major cellular fatty acids of strain DQS-4(T) were summed feature 3 (comprising C16 : 1�s7c and/or C16 : 1�s6c), C16 : 0 and C18 : 1�s7c. The major cellular hydroxy fatty acid was C10 : 0 3-OH. The DNA G+C content was 64.5 mol%. The polar lipid profile consisted of phosphatidylethanolamine, phosphatidylglycerol, diphosphatidylglycerol and several uncharacterized aminophospholipids and phospholipids. The mean level of DNA-DNA relatedness between strain DQS-4(T) and A. indigens LMG 9092(T) was 27.4 %. On the basis of the genotypic and phenotypic data, strain DQS-4(T) represents a novel species in the genus Azoarcus, for which the name Azoarcus olearius sp. nov. is proposed. The type strain is DQS-4(T) (= BCRC 80407(T) = KCTC 23918(T) = LMG 26893(T)).