Japanese flounder were reared by a seawater recirculation and filtration system with immobilized bacteria. As a water purification system, used were a nitrification tank and denitrification tank which were both filled with immobilized nitrifying bacteria and denitrifying bacteria, respectively. The flounder could be reared for 180 days with their body weight doubled at the end of experiment. Ammonia exhausted from the flounder was not accumulated in the rearing water. Nitric acid produced by the oxidation of ammonia was prevented from heightening its concentration in the denitrification tank. Usable immediately after having been filled into the filtration tank, both immobilized nitrifying and denitrifying bacteria are stable in activity without being affected by organic matter in the rearing water. The dissolution and disintegration of carrier were not confirmed. The high activity of immobilized nitrifying bacteria and denitrifying bacteria was observed on the surface and in the central part, respectively of the carrier. The activity of both bacteria was confirmed inside the carrier. From the foregoing, the high density rearing was known to be possible even in a filtration tank miniaturized to the size of about 20% of the conventional one. 29 refs., 18 figs., 2 tabs.