Using UV, CD, and NMR, we demonstrate that the important bacterial signaling molecule involved in biofilm formation, cyclic diguanosine monophosphate (c-di-GMP), exists as a mixture of five different but related structures in an equilibrium that is sensitive both to its concentration and to the metal present. At the lower concentrations used for UV and CD work (0.05-0.5 mM), Li+, Na+, Cs+, and Mg2+ favor a bimolecular self-intercalated structure, while K+, Rb+, and NH4+ favor formation of one or more guanine quartet complexes as well. At the higher NMR concentrations (similar to 30 mM), the bimolecular structures associate and rearrange to a mixture of all-syn and all-anti tetramolecular and octamolecular quartet complexes. With K+ the octamolecular complexes predominate, while with Li+ the tetramolecular and octamolecular quartet complexes are present in approximately equal amounts, along with the bimolecular structure. We also find that both guanine amino groups in c-di-GMP are essential for formation of the quartets, because substitution of inosine for one guanosine allows formation of only the bimolecular structure. Further, two molecules of c-di-GMP tethered together are constrained in such a way that limits their ability to form these quartet complexes. The polymorphism we describe may provide different options for this signaling molecule when performing its functions in a bacterial cell, with K+ and its own local concentration controlling the equilibrium.