The effects of binding calcium ions to the double helical forms of gramicidin present in methanol solution were examined using circular dichroism (CD) and nuclear magnetic resonance (NMR) spectroscopies. It was found that calcium ions principally alter the relative composition of the equilibrium mixture of gramicidin conformers present in the solvent. In the absence of calcium, both parallel and antiparallel double helices are present. However, the addition of small amounts of Ca2+ shifts the equilibrium towards the left-handed parallel double helical form. This conformational change prevents monovalent cations (caesiums) from binding to the gramicidin double helix, and even converts the shorter, wider anti-parallel double helical form normally produced in the presence of caesium into the longer, narrower parallel double helical form. Furthermore, a temperature study showed that calcium ions tend to stabilize this form relative to the ion-free forms. The conformation of gramicidin is further changed, becoming a disordered structure, when the concentration of Ca2+ is raised. Thus, the binding of divalent calcium ions has a number of dramatic effects on the conformations of gramicidin present in solution.