A new marine ammonia-oxidizing bacterium (strain TNO632) was isolated in a screening of 80 sea-water samples on gellan gum-solidified plates, The cells of strain TNO632 are rods (0.7-0.9 x 1.0-2.0 mu m), gramnegative, 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 present. The G + C content of the total DNA is 53.3 mol%. The similarity of 16S rRNA (%) to Nitrosomonas europaea ATCC 25978T (type strain) is 94.26%. The optimum temperature for maximum growth in marine ASW-medium is 27-29 degrees C. The optimum concentrations of ammonium sulfate, sodium chloride, and magnesium chloride for growth are 26, 100 and 100 mM, respectively. Growth of the strain is impossible when urea is used as the sole carbon source. Hydroxylamine cytochrome c reductase (HCR) and triosephosphate isomerase (TIM) were detected and found to possess high specific activities (980 and 140 U/mg, respectively) in TNO632 compared to those in ATCC 25978T (450 and 14 U/mg).