This paper used a new entrapment method for cell immobilization to elucidate the rate of autotrophic denitrification and obtain the appropriate operating conditions for drinking water treatment. Alcaligenes eutrophus, a hydrogenotrophic denitrifier, was immobilized in polyacrylamide and alginate copolymer to evaluate denitrification in continuous mode and batch mode in a fluidized-bed reactor. The total nitrogen removal rate in a continuous test was increased with operation time and reached a maximum rate (0.6-0.7 kg-N/m(3)/day) after 6 days. The dissolved hydrogen concentration had a significant effect on denitrification. In batch test, nitrite reductase was inhibited when the dissolved hydrogen concentration fell below 0.2 mg/l, while nitrate reductase was inhibited at a concentration below 0.1 mg/l. The phosphate concentration also affected denitrification, especially in the accumulation of nitrite. The bacteria have a good ability to adapt to a shock nitrate loading. A high nitrite concentration was found when nitrate was first added to the reactor in batch mode. The nitrite concentration, however, decreased significantly with the second and third addition of nitrate after its degradation in the first addition.