Fluorescence resonance energy transfer (FRET) occurred readily in a cholate hexamer labeled with a naphthyl donor and a dansyl acceptor at the chain ends when the hexamer was solubilized by sodium dodecyl sulfate (SDS) micelles in water. Independence of the energy transfer efficiency over 1−70 mM SDS suggested that the energy transfer resulted from the folding of the hexamer instead of its intermolecular aggregation within the micelle. Upon addition of sodium chloride to the solution, energy transfer became less efficient, indicating unfolding of the oligocholate. In contrast, the oligocholate stayed folded in the micelle of nonionic Brij 30, in the presence or absence of NaCl. These results suggested that the oligocholate preferred to fold within the small spherical SDS micelles but unfold when the preference for spherical over rodlike micelles was not strong enough to overcome the tendency for the oligocholate to unfold.