Sodium-potassium ATPase is an ATP-powered ion pump that establishes concentration gradients for Na+ and K+ ions across the plasma membrane in all animal cells by pumping Na+ from the cytoplasm and K+ from the extracellular medium. Such gradients are used in many essential processes, notably for generating action potentials. Na+, K+-ATPase is a member of the P-type ATPases, which include sarcoplasmic reticulum Ca2+-ATPase and gastric H+, K+-ATPase, among others, and is the target of cardiac glycosides. Here we describe a crystal structure of this important ion pump, from shark rectal glands, consisting of - and -subunits and a regulatory FXYD protein, all of which are highly homologous to human ones. The ATPase was fixed in a state analogous to E2·2K+·Pi, in which the ATPase has a high affinity for K+ and still binds Pi, as in the first crystal structure of pig kidney enzyme at 3.5 Å resolution. Clearly visualized now at 2.4 Å resolution are coordination of K+ and associated water molecules in the transmembrane binding sites and a phosphate analogue (MgF42-) in the phosphorylation site. The crystal structure shows that the -subunit has a critical role in K+ binding (although its involvement has previously been suggested) and explains, at least partially, why the homologous Ca2+-ATPase counter-transports H+ rather than K+, despite the coordinating residues being almost identical.