The structural parameters of Hg2+ hydration were studied in 0.225 mol/L solutions of Hg2+ in DNO3/D2O by means of neutron diffraction with isotopic substitution of Hg-199 for Hg-nat. It was found that Hg2+ is hydrated by a first solvation shell of six water molecules. The observed Hg-O and Hg-H distances are equal to 2.48 +/- 0.05 and 3.08 +/- 0.05 A, respectively. The angle phi between the plane of the water molecule and the cation-water oxygen axis is similar to 35 degrees. The solvation of Hg2+ therefore mimics very closely that of Ca2+ (the Ca-O and Ca-H distances are 2.40 and 3.03 A, respectively) and helps to account for the extreme toxicity of mercury(II). We note also that the Hg-O distance obtained in the neutron diffraction experiment is larger by similar to 0.1 A than that obtained by X-ray diffraction. This difference is consistent with a shift of the oxygen electron density toward the mercury cation due to the covalency of the Hg-O interaction.