Na +,K +-ATPase is a heterodimer of α and β subunits and a member of the P-type ATPase family of ion pumps. Here we present an 11-Å structure of the heterodimer determined from electron micrographs of unstained frozen-hydrated tubular crystals. For this reconstruction, the enzyme was isolated from supraorbital glands of salt-adapted ducks and was crystallized within the native membranes. Crystallization conditions fixed Na +,K +-ATPase in the vanadate-inhibited E 2 conformation, and the crystals had p1 symmetry. A large number of helical symmetries were observed, so a three-dimensional structure was calculated by averaging both Fourier-Bessel coefficients and real-space structures of data from the different symmetries. The resulting structure clearly reveals cytoplasmic, transmembrane, and extracellular regions of the molecule with densities separately attributable to α and β subunits. The overall shape bears a remarkable resemblance to the E 2 structure of rabbit sarcoplasmic reticulum Ca 2+-ATPase. After aligning these two structures, atomic coordinates for Ca 2+-ATPase were fit to Na +,K +-ATPase, and several flexible surface loops, which fit the map poorly, were associated with sequences that differ in the two pumps. Nevertheless, cytoplasmic domains were very similarly arranged, suggesting that the E 2-to-E 1 conformational change postulated for Ca 2+-ATPase probably applies to Na +,K +-ATPase as well as other P-type ATPases.