To clarify the optimal environmental conditions for the ark shell Scapharca kagoshimensis in the brackish lagoon Nakaumi, its fi ltration and survival rates under various salinity（9.4-31.4 psu）and temperature（8-31d）were examined by laboratory experiments. The fi ltration rate varied with varying salinity and temperature conditions. The rate was highest at intermediate salinity（18.8-22.0 psu）and lowest at low salinity（9.4 psu）. Under the low salinity condition, fi ltration was not observed. In addition, the fi ltration rate was highest at summer temperature（27d）and lowest at winter one（8d）. The survival rate also varied with varying salinity and temperature conditions. Dead individuals were only observed under the combination of hot summer temperature（31d）and low salinity（9.4-15.7 psu）conditions. In particular, the combination of 31d and＜12.2 psu was critical for the shell, the survival rates were 0％ and 25％ at 9.4 psu and 12.2 psu, respectively. In the lagoon, where halocline is maintained whole through the year, the critical temperature and salinity for the shell’s filtration activity and survival were only recorded in shallow zones above halocline. Therefore, it is suggested that shallow zones above halocline is not suitable habitat for the ark shell.