Eukaryotic proteins are targeted for degradation by covalent ligation of multiubiquitin chains. In these multiubiquitin chains, successive ubiquitins are linked by an isopeptide bond involving the side chain of Lys48 and the carboxyl group of the C-terminus (Gly76). The crystal structure of a tetraubiquitin chain (Ub4) has been determined and refined at 2.4 A resolution. The molecule exhibits both translational and 2-fold rotational symmetry; each pair of (rotationally symmetric) ubiquitin molecules in Ub4 is related to the next pair by a simple translation. The 2-fold symmetry in each pair of ubiquitin molecules is quite different from the 2-fold symmetry observed in the previously determined structure of isolated diubiquitin. There are multiple hydrophilic contacts among the four ubiquitin molecules, but the hydrophobic surface formed in the middle of diubiquitin is not seen. The structure of the tetraubiquitin chain demonstrates how a multiubiquitin chain of any length can be formed.