Aquaporin-1 is a mater channel found in mammalian red blood cells that is responsible for high water permeability of its membrane, Our electron crystallographic analysis of the three-dimensional structure of aquaporin-1 at 4.5-Angstrom resolution confirms the previous finding that each subunit consists of a right-handed bundle of six highly tilted transmembrane helices that surround a central X-shaped structure, in our new potential map, the rad-like densities for the transmembrane helices show helically arranged protrusions, indicating the positions of side chains. Thus, in addition to the six transmembrane helices, observation of helically arranged side-chain densities allowed the identification of two short alpha-helices representing the two branches of the central X-shaped structure that extend to the extracellular and cytoplasmic membrane surfaces, The other two branches are believed to be loops connecting the short alpha-helix to a neighboring transmembrane helix. A pore found close to the center of the aquaporin-1 monomer is suggested to be the course of mater ham with implications for the water selectivity.