An ammonia-oxidizing bacterium, strain TCH716, was isolated from alkaline soil at Harbin city, China. The cells of strain TCH716 are lobate (0.8-1.5 x 1.0-2.0 mum), gram-negative, obligately aerobic, and nonmotile. Colonies (1-2 mm in diameter) on gellan gum plate culture are reddish, circular, and smooth. The G+C content of DNA is 54.78 mol%. Its percentage of 16S rRNA gene sequence similarity (%) to Nitrosolobus multiformis ATCC 25196(T) (type strain) is 98.56%. This bacterium has an optimal growth temperature and pII at 30 degreesC and 8.0-8.5, respectively. The concentration of ammonium sulfate in the HEPES medium for optimum growth of this bacterium is 38 mM. Strain TCH716 was found to have a plasmid (approximately 6.5 kbp) that possessed a plasmid-linked gene for sulfonamide resistance. Phosphoglycerate kinase, RubisCO and PEPC were found to possess high specific activities compared to the activities of these enzymes in strain ATCC 25978(T) In identification of strain TCH716, both morphological characteristics (compartmentalized cells) and the phylogenetic relationship based on 16S rRNA gene sequence are important. Based on results obtained, strain TCH716 belongs to the genus Nitrosolobus, and designated as Nitrosolobus sp. TCH716.