It was found for the first time that rich lamellar structures are formed by a single-tailed amphiphilic ionic liquid (IL), 1-dodecyl-3-methylimidazolium β-naphthalene sulfonate ([C12mim][Nsa]), in aqueous solutions without any additives. With increasing IL concentration, spontaneous transition from micelles to unilamellar and multilamellar vesicles, planar bilayers, and then to lamellar liquid crystals occurs.