A self-assembly of a helical peptide in an aqueous medium was investigated in an attempt to compose a supramolecular structure. A peptide, HA16B, having a hydrophobic a-helical body and a hydrophilic polar head was synthesized and hydrated in water by sonication to obtain a transparent dispersion. Circular dichroism measurement showed that the peptide took an a-helical conformation in the dispersion. Transmission electron microscopy and dynamic light scattering measurement revealed that spherical assemblies were formed with an extremely narrow distribution of size (diameter in the range of 30-40 nm), which is much larger than the molecular length of the peptide. These observations strongly suggest that the peptide assembly took a liposome-like vesicular structure.