Erabutoxin a has been crystallized in its monomeric and dimeric forms. The structures were refined at 1.50 and 1.49 A resolution, respectively, using synchrotron radiation data. The crystals belong to space group P212121, with cell dimensions a = 49.84, b = 46.62, c = 21.22 A for the monomer and a = 55.32, b = 53.54, c = 40.76 A for the dimer. Using starting models from earlier structure determinations, the monomeric structure refined to an R value of 16.7% (8004 unique reflections, 17.0-1.50 A resolution range), while the dimeric structure has been solved by the molecular-replacement method with a final R value of 16.9% (19 444 unique reflections, 17.4-1.49 A resolution range). The high-resolution electron-density maps clearly revealed significant discrete disorder in the proteins and allowed an accurate determination of the solvent structure. For the monomer, the side chains of six residues were modelled with alternate conformers and 106 sites for water molecules and one site for a sulfate ion were included in the final model, whereas for the dimer, 206 sites for water molecules were included and both C-terminal residues together with the side chains of 11 residues adopted alternative conformations. A comparison was made with earlier structure determinations. The features of the solvent structure of the erabutoxin molecules are discussed in detail.