A new psychrotrophic ammonia-oxidizing bacterium (strain TYM9) was isolated in a screening of 150 soil samples on a gellan gum-solidified plate. The cells of strain TYM9 are slender cuved rods (0.2-0.4 x 1.0-3.0 mu m), gram negative, obligately aerobic and nonmotile. Colonies (1-2 mm in diameter) on a plate culture are reddish, circular and smooth. Intracytoplasmic membranes characteristic of nitrifying bacteria are absent. The G + C content of the DNA is 53.0 mol%. The optimum temperature for maximum growth in HEPES medium is 20 degrees C and minimum growth occurs at 5 degrees C and 35 degrees C. The optimum concentration of ammonium sulfate for growth is 20 mM. Growth of the strain is possible in the presence of cephaloridine (500 mu g/ml). Hydroxylamine oxidase (HAO) was detected with high specific activity (346 U/mg) compared to that for N. europaea ATCC 25978T (4.5 U/mg). The level of RuBisCO activity in strain TYM9 is similar to that in strain ATCC 25978T. Strain TYM9 has morphologically and physiologically different characteristics from those of Nitrosovibrio tenuis and members of the genus Nitrosomonas. Strain TYM9 was identified as belonging to the genus Nitrosovibrio, and assigned the Nitrosovibrio sp. TYM9.