A study of the quaternary structure of hemocyanin from the scorpion Leirus quinquestriatus was carried out. The amino acid composition and the copper content were determined. For the native molecule, a sedimentation coefficient of 36.2 S and a molecular weight of 1.74 x 106 were found. In sodium dodecyl sulfate-polyacrylamide gel electrophoresis, five bands were obtained. Most of the material migrated with a mobility corresponding to a molecular weight of 74 x 103. Dissociation of the molecule at alkaline or acidic pH yielded a 5S component corresponding to individual polypeptide chains. At the transition pH values, intermediate products of dissociation with sedimentation coefficients of 15.4 and 23.9 S, attributed to quarter- and half-molecules respectively, were obtained. Electron micrographs of the native molecule showed two structures, a rectangle and two rhombi diametrically connected together at the corners of the obtuse angle. We propose a model for the molecule consisting of 24 spherical subunits, each subunit representing a 5S particle. Every six subunits are organized in an octahedral arrangement representing a 16S unit, and the 36S molecule is a square-planar structure composed of four 16S units. The square arrangement of the 16S units is consistent with an analysis of the hydro-dynamic data by the Kirkwood treatment, and projections of the model agree with particle profiles observed in the electron micrographs. Implications of the proposed structure on the observed heterogeneity of monomeric 5S subunits are discussed.