The two independent NMR experiments were performed to investigate the interaction between CaCl2 and the gramicidin A (GA) ion transport channel, using 13C-enriched GA and GA molecules incorporated into dodecylphosphocholine (DPC) micelles. The chemical shifts of C-13 labeled carbonyl carbons vs. CaCl2 concentration demonstrate that Ca2+ and Cl- ions interact as an ion pair within the GA structure with the Cl- ion located near the position of the carbonyl group of the Trp11 residue some 5.5 A from the mouth of the GA helix, and the Ca2+ ion bound at the position of the carbonyl group of the Trp15 residue some 2.5 A from the entrance to the helical pore. The measurements of the 35Cl line-widths and transverse relaxation times illustrate that the interaction occurs between Cl- ions and GA in DPC when in CaCl2 solution, that no interaction is detected between Cl- ions and GA in DPC when in NaCl solution, and that the interaction between Cl- ions and GA in DPC when in MgCl2 solution is much weaker than in CaCl2 solution. In short, a Cl- ion can enter the GA when it is paired with a divalent Ca2+ ion; and Ca2+ and Cl- ions as a pair exchange rapidly with sites of the GA dimer.