AbstractTransmission electron microscope and X-ray microanalyser studies were carried out on the cells of mercury accumulating aquatic liverwortJungermannia vulcanicolafrom Kashiranashigawa Stream.The mercury which is accumulated from stream water is mainly located in the cell wall in the form of electron dense particles having the size range 0.01–0.1μm up to 0.2μm. The chemical form of the mercury in the electron dense particles is a mercury-sulphur compound, probably mercuric sulphide, which is not toxic to living organisms.