The frequency dispersions of the water O-17 and H-2 nuclear magnetic relaxation rates have been measured in solutions of the calcium-binding protein calbindin D-9k in the apo and calcium-loaded states. The relaxation data show that the residence times of the two water molecules that ligate calcium ions in the crystal structure are in the range 5 ns to 7 mu s, much longer than for calcium-coordinated water in bulk solution. In addition to a twist libration of substantial amplitude, the calcium-coordinated water molecules in calbindin undergo a fast (<1 ns) flip motion, resulting in a drastic reduction of the H-2 dispersion amplitude. The residence time as well as the internal motions of these water molecules are largely governed by strong hydrogen bonds to side chains that may be essential for the cooperativity of calcium binding. In addition to the calcium-coordinated waters, calbindin contains (at least) one long-lived (>5 ns) water molecule, which we tentatively identify as a structure-stabilizing water molecule buried in a surface pocket near the linker loop. Even at pD 6.7, the H-2 relaxation dispersion is dominated by rapidly exchanging carboxyl deuterons in highly ordered side chains. The present study provides the first direct observation by means of NMR of water molecules coordinated to a diamagnetic metal ion in a protein solution.