A two-dimensional (2D) analogue of 3D fibrous supramolecular assemblies was formed spontaneously on the water surface by a short chain carboxyazobenzene derivative. It is a mesoscopic tape 1 nm thick, 80 nm wide, and can be as long as 0.1 mm. The tapes are obtained as Langmuir-Blodgett (LB) films on mica and examined by atomic force microscopy (AFM). AFM images exhibit complex internal structures, indicating that the tapes are not single crystals. They are not produced by LB processes, such as compression and lifting, and are not induced by the mica surface. The mesoscopic tape is self-contained and exists independent of surface pressure whose equilibrium value is nearly zero. UV-vis reflection spectroscopy on the water surface shows a peak at 305 nm. Increasing the water temperature to above 30 degrees C or leaving the film on the water surface for 14 h caused the tapes to disintegrate into small clusters and the UV-vis peak to shift to 365 nm. The concentration at which the solvent evaporates controls the tape formation. At low concentrations, the tapes coexist with the domains of an ordinary phase. Then, there is a critical aggregation concentration at 8 nm(2)/molecule that the tape fraction increases abruptly. These results suggest that the tape is closely related to associations of 2D clusters.