Nitrogen compounds in seawater are now contributing to serious water pollution problems. In this study, continuous removal of nitrogen in seawater using nitrifiers and denitrifiers immobilized in macro-porous cellulose carrier “AQUACEL” was examined. In nitrification, the nitrite oxidation step becomes the rate limiting step unless an influent inorganic carbon (g) / influent NH4-N (g) ratio (IC/NH4-N) of 7.23 is maintained. This is equivalent to an influent alkalinity (g) / influent NH4-N (g) ratio (ALK/NH4-N) of 8.25. Nitrite oxidizers were also sensitive to change in NH4-N loading. Unlike other biological removal systems used for seawater, trace element solution (containing Mo, Cu, Mn, Co, Fe, etc.) was added only at a high NH4-N loading rate of 0.65 kg-N/m3/carrier/d (at NH4-N concentration of 40 g/m3) and acclimatizing period was short, i.e., about a week. The maximum NH4-N loading rate obtained which removed 99 to 100% of the nitrogen compounds, was 1.30 kg-N/m3/carrier/d. For completion of denitrification, an influent phosphorus (g) per influent NO3-N(g) ratio (P/NO3-N) of 0.03 was required. Trace element solution (containing Fe, Mn, Mo, etc.) was doubled to 0.02% at NO3-N concentration of 560 g/m3. In addition, methanol concentration must be maintained at 30% more of the theoretical value of carbon concentration requirements. Copper enhanced nitrite reduction at an influent Cu(g) per influent NO3-N(g) ratio (Cu/NO3-N) of 0.002. The maximum allowable NO3-N loading rate necessary to remove about 99 to 100% of the nitrogen compounds was 20.79 kg-N/m3/carrier/d. This study reveakls that the AQUACEL system has high nitrifying and denitrifying capacities. The nitrogen loading capacity of denitrification is about ten times that of nitrification and is comparable to that of freshwater which also employed the AQUACEL system. In contrast, nitrogen loading capacity of nitrification is about six times less than that of freshwater, which indicates a higher sensitivity of nitrifiers to salinity. This indficates high sensitivity of the immobilized nitrifying bacteria to salinity. Morphological observations show that the ammonia oxidizers are a mixed culture ofNitrosomonas spp . and Nitrosovibrio spp., while the nitrite oxidizer is a Nitrobacter spp. The immobilized denitrifying bacteria showed similar morphological characteristics to the Hyphomicrobium spp.