Single crystals of the lead zirconate-titanate solid solution containing 85 percent and those containing less than 40 percent of lead zirconate have been prepared from a molten solvent of a mixture of KF and PbF 2 , and the measurements of optical birefringence have been made. They are all optically negative. The absolute value of birefringence of the crystal containing less than 20 percent of PbZrO 3 shows a maximum when plotted against temperature and resembles the behavior of PbTiO 3 . Such an anomalous behavior tends to vanish with increasing amount of PbZrO 3 . These phenomena are explained taking account of the effect of covalent bonding, after the method developed by Kobayashi and Yamada. 1) The contribution of the covalent nature increases at first and then decreases gradually with increasing PbZrO 3 content. The crystal containing 85 percent of PbZrO 3 is in rhombohedral phase and has larger anisotropy than in tetragonal phase.