An ammonia-oxidizing bacterium (strain YNSRA) was isolated from the rhizoplane of the reed (Phragmites communis) used in an aquaponics plant which is a wastewater treatment plant.. Strain YNSRA was identified as Nitrosomonas communis by taxonomic studies. The hydroxylamine-cyto- chrome c reductase (HCR) of strain YNSRA was found to have a higher activity (25.60 u/mg) than that of Nitrosomonas europaea ATCC25978(T) (8.94 u/mg). Ribulose-1.5-bisphosphate carboxylase (RubisCO) activity was detected at very low levels in strain YNSRA, whereas strain ATCC25978(T) had definite activity.