Riboswitches harness the structural and dynamic sophistication of RNA to coordinate specific ligand recognition with changes in gene expression. Design of molecules to manipulate riboswitch responses relies on our understanding of their RNA-ligand interactions. Here we demonstrate that for the adenine (A) riboswitch (ARNA) these interactions are highly dynamic. Given that 2-aminopurine (Ap) mimics A in its interactions with ARNA, we use the fluorescence lifetime of Ap to interrogate individual Ap-ARNA conformers (dynamic exchange times > similar to 10 ns). Counter to predictions of two state and induced fit models, the ligand-bound A riboswitch is not a single, highly ordered structure: We detect at least three distinct Ap-ARNA conformers in ensemble solution. Their distribution indicates that they are not high-energy RNA folding intermediates but are instead energetically similar (Delta G < 1 kcal mol(-1)) conformers whose thermal stability, ligand, and Mg2+ binding affinity differ substantially. Our experimental characterization suggests that these conformers are structurally distinct locally at the ligand binding site and globally in the arrangement of the P2-P3 stems. These results correlate well with recent single molecule characterization of P2-P3 end-to-end distances and exchange rates, but contrast with recent NMR results which suggest that the highly homologous G riboswitch exhibits a static global structure both with and without ligand. These distinct dynamics may well be the root of the divergent specificity and function of the A and G riboswitches. We predict that conformational dynamics within the bound A riboswitch underlie its regulatory responses and that these dynamics are directed by ligand structure.