Minimum beam electron microscopy and the three-dimensional reconstruction technique were used to analyse negatively stained Alfalfa mosaic virus particles and polymers of reassembled coat protein. Three strains of the virus were investigated: strains 425, 15/64 and VRU. The quaternary structure of the protein coat is based on a hexagonal net; in the VRU strain the orientation of this P6 lattice with respect to the cylindrical axis is not unique, so that different structures exist. In all strains the so-called “stacked” form is present, whereas in the VRU strain the “helical” forms of the lattice also appear. The hexagonal cell dimension is 84 Å. In those instances where cylindrical structures are formed by reassembling coat protein with or without the presence of nucleic acid, the quaternary structure of these products is similar to the parent virus structure. Image reconstruction shows that at low resolution (30 Å) the density distribution in negatively stained VRU virus particles, reassembled VRU coat protein tubes and VRU coat protein structures assembled in the presence of nucleic acid is similar. The protein is distributed in the P6-cell so that stain-accumulating centres exist at the symmetry positions, especially at the outside part of the cylindrical structure. The data suggest that the coat protein molecules are arranged in a relatively open network.