The fresh water teleost, Oryzias latipes (the wild type) possesses in the dermis three kinds of chromatophores : the. melanophores, the xanthophores and the guanophores. The melanophores are the most conspicuous type of the chromatophores in the teleosts and many studies on the fish chromatophores have been done in these cells. It has been well accepted that the melanophores in many of the teleosts are controlled under the autonomic nervous system. In excised preparations, melanophores and xanthophores are in expanded state and guanophores in contracted one in physiological solution. When the perfused solution is exchanged from physiological solution to isotonic KCl or adrenaline solution, the melanophores and the xanthophores cdntract and the guanophores expand within a few minutes. Namely, the melanophores and the xanthophores respond in the same manner to stlmulant ions and drugs and the guanophores behave with a reversed response. Miyoshi (1952) studied the response of the guanophores of Oryzias latipes to some chlorides. However, much less attention has been paid to the physiology of the yellow cells, no doubt because their appeal as a subject for study suffers from their relatively poor visibility. In this paper, the responses to potassium ions of the xanthophores in an isolated scale were examined and the mode of action in this ion was discussed.