When the cells of the newly isolated marine bacterium, Vibrio alginolyticus, were inoculated on to an inorganic packing material in biofilter, and a load of ammonia of 2.4-22.5 g-N kg(-1) dry packing material was introduced continuously under non-sterile conditions, the average amount of NH3 removed exceeded 85% over 61-d operation. The maximum removal capacity and the complete removal capacity were 22.8 g-N kg(-1) dry packing material d and 18.6 g-N kg(-1) dry packing material d, respectively, which were about four times larger than those obtained in autotrophic nitrifying sludge inoculated on the same packing material.