The membranes of a double-chain amphiphile dioctadecyl-N-[4-[[p-(N,N-dimethylamino)phenyl]azo]- pyridiniumbutyryl]-L-aspartate bromide (DDPA) were investigated by atomic force microscopy (AFM) and UV-vis absorption spectroscopy. The line-shaped aggregates of the molecules were found in the Langmuir-Blodgett (LB) monolayers of DDPA. The widths of these lines were about twice the length of the molecule. A semicylindrical structure was proposed to account for the observation. It was found that the DDPA molecules self-organized into bilayers and vesicles when the LB film was immersed in aqueous solutions. The bilayer showed good compressibility. The mixed DDPA and alpha-L-dipalmitoylphosphatidylcholine LB films were also examined, and the phase separation was observed. From the in situ AFM observation, the self-organization process was found to occur in the DDPA region. The mechanism of self-organization in aqueous solutions was also discussed.