Free-living amoebae of the genus Acanthamoeba (Gr II) isolated from a potable water reservoir harbored intracytoplasmic multiplying microorganisms of unknown nature. Their voluminous envelopes exhibit a very fine texture. This texture results from tubules with central core. In some longitudinal sections ostiolar gaps were found at one of the poles. The tubules forming the ostiole lack the central core and leave wider spaces than the tubules of the envelope. The interior of the cell appears nearly amorphous except for a delicate balloon like invagination of the basis of an ostiole and a dot like structure observed near one of the poles. Because of some of these characteristics the unique organisms are supposed to be related distantly to archaea.