The structure of tetragonal hen egg-white lysozyme soaked in a periodate solution has been determined to a resolution of 1.8 A. Four high-occupancy periodate positions have been identified on the basis of the anomalous signal of the I atoms. The four periodates exhibit a regular rectangular arrangement on the surface of the lysozyme molecule. No similar regular arrangement was found either in lysozyme crystals soaked in other heavy-atom anions or in other structures from the Protein Data Bank. Depending on their position on the surface of the protein, the periodate ions deviate to a varying extent from ideal octahedral geometry.