To determine the best type of carrier for marine nitrification, the macro-porous cellulose and polyester carriers were compared in continuous nitrification experiments using a 15.7 dm(3) airlift-type bioreactor. Employing a feeding medium containing only ammonium chloride (as substrate) and sodium bicarbonate (as carbon source), ammonium loading rates of 2.6 and 0.65 kg-N/m(3)-carrier/d were obtained for polyester and cellulose carriers, respectively. The addition of a trace element solution (TE) resulted in a further increase of ammonium loading rates to 5.20 and 1.62 kg-N/m(3)-carrier/d, for the polyester and cellulose carriers, respectively. Nitrite oxidation became a rate-limiting step unless a minimum influent of inorganic carbon (g)/influent NH4-N (g) ratios of 1.2 and 5.0, and 0.02% TE at 50 and 40 g/m(3) NH4-N concentration were maintained for the polyester and cellulose carriers, respectively. These differences in nitrification capacity and nutrient requirements are due to differences in the physico-chemical properties of the carriers. Measurement of the effectiveness factor showed that in cellulose carriers, the diffusion of oxygen was limited. In addition, precipitation, which is common with seawater, occurred inside the cellulose carriers after approximately five months of continuous experimental runs. The precipitates further limited oxygen and other nutrients＇ transfer, and also caused inefficient circulation of the heavier cellulose carriers. Morphological observations show that the ammonia and nitrite oxidizers are Nitrosomonas marina and Nitrospina sp., respectively.