Creation of tubular structures through the self-assembly of macrocyclic molecules has gained great attention in the chemical, biochemical, and material sciences. Through a designed introduction of two octaethylene oxide and two diacetylene moieties, we prepared a macrocycle MCDA BisOEG, which is water-soluble and photopolymerizable and displays a lower critical solution temperature (LCST) behavior. The hydrodynamic diameter of MCDA BisOEG, measured by dynamic light scattering method, was ca. 6.5 rim at 25 degrees C and increased sharply to ca. 2 mu m at temperatures above 34 degrees C (LCST). Below the LCST, the macrocycle in aqueous solution formed tubular structures, which upon UV irradiation generated blue conjugated polydiacetylene (PDA) nanotubes. The PDA nanotubes undergo a blue-to-red color change when heated above the LCST. No polymerization occurs above the LCST owing to the disordered aggregation of the diacetylene monomer. This is the first example of the preparation of a macrocycle-based, thermoresponsive, conjugated polymer nanotube in aqueous solution.