Abstract：Recently, atmospheric-pressure nonequilibrium plasmas called "cold plasmas" have received much attention in view of their environmental and bio-medical applications. In cold plasma, various radical species such as atomic oxygen and OH are generated and these radicals play an important role in degradation of toxic organic compounds and sterilization of bacteria and fungi. So, many kinds of cold plasma devices have been developed so far. In this study, the cold plasma generated in contact with water was used to chlorophenols degradation and the influence of the plasma treatment was evaluated by means of the Daphnia magna acute immobilisation test. The cold plasma irradiation on the M4 medium which is the breeding water for Daphnia magna was also conducted to check the toxic effect of plasma treatment. キーワード： オオミジンコ, 低温プラズマ, 急性遊泳阻害試験