Acclimated marine nitrifying sludge (AMNS) prepared from activated sludge obtained from a night soil treatment plant equipped with a sea water dilution system for controlling the reactor’s temperature was successfully immobilized using a polyvinyl alcohol (PVA) freezing method. About 2 weeks of recovery culture was required before continuous treatment could be carried but. Transmittance electron microscopic observations of sliced immobilized AMNS pellets indicated that bacteria with an intracytoplasmic membrane dominated the AMNS colony. Continuous nitrification experiments of NH4-N containing synthetic sea water were carried out in a 1.2 l bioreactor containing 57.2 g (wet weight) of immobilized AMNS pellets. The NH4-N removal rate reached a saturation level above an NH4-N loading rate of 1.5 mg-NH4-N/g-pellet/d. The maximum allowable NH4-N loading rate necessary to obtain 90% NH4-N removal was found to be 1.0 mg-NH4-N/g-pellet/d, It was possible to store the immobilized AMNS pellets in a refrigerator for at least 1 week without loss of nitrifying capability. Inorganic carbon source was shown to be a limiting factor in the continuous nitrification experiments. The maximum allowable NH4-N loading rate needed to obtain 90% NH4-N removal increased to 2.0 mg-NH4-N/g-pellet/d through supplementation of an inorganic carbon (IC) source to the influent synthetic sea water.