Many eukaryotic cellular and viral proteins have a covalently attached myristoyl group at the amino terminus. One such protein is recovering a calcium sensor in retinal rod cells, which controls the lifetime of photoexited rhodopsin by inhibiting rhodopsin kinase(1-6). Recoverin has a relative molecular mass of 23,000 (M-r 23K), and contains an amino-terminal myristoyl group (or related acyl group) and four EF hands(7). The binding of two Ca2+ ions to rccoverin leads to its translocation from the cytosol to the disc membraned(8,9). In the Ca2+-free state, the myristoyl group is sequestered in a deep hydrophobic box, where it is clamped by multiple residues contributed by three of the EF hands(10). We have used nuclear magnetic resonance to show that Ca2+ induces the unclamping and extrusion of the myristoyl group, enabling it to interact with a lipid bilayer membrane. The transition is also accompanied by a 45-degree rotation of the amino-terminal domain relative to the carboxy-terminal domain, and many hydrophobic residues are exposed. The conservation of the myristoyl binding site and two swivels in recoverin homologues from yeast to humans indicates that calcium-myristoyl switches are ancient devices for controlling calcium-sensitive processes.