Preparations of purified (Na+ + K+)-ATPase contain both fragments of membranes and long and undulating cylindrical structures. These structures have been described as edgeways of membrane fragments. We have analyzed these structures using negative staining, thin sectioning and freeze-fracture-etch electron microscopy and describe their structure for the first time. Each cylinder is 12-19 nm in width and is comprised of an unstained core from which rows of distinct particles spaced 5-6 nm apart project on both sides. Each cylindrical structure was interpreted as a linear polymer of (alpha beta)2 dimers of (Na+ + K+)-ATPase molecules. Therefore, the particles that project from both sides are the cytoplasmic domains of the molecules of the enzyme, whereas the membrane-spanning domains form the unstained core of the cylinder. From considerations of the packing of the dimers in the cylinder we conclude that the cross-sectional area of the cytoplasmic domain should be larger than that of the membrane-spanning domain. Our results are consistent with the hypothesis that the (alpha beta) protomer is the native state of the enzyme. The (alpha beta)2 dimers observed in the fractions are the result of a secondary aggregation process occurring during the purification procedure.