The migration of boundaries between single crystal and polycrystalline ZnO was investigated using single crystals with well-defined crystallographic faces. The migration rate of the basal (0001) planes through polycrystalline ZnO depended on the crystallographic polarity of the basal plane. Grain boundary migration in the [0001] direction was much faster than in the [0001] direction. Migration rates of boundaries in nonpolar directions were somewhat slower than that in the [0001] direction. Slow growth in the [0001] direction and rapid growth in nonpolar directions found in the present study help to explain the anisotropic growth of the grains that contain head-to-head inversion boundaries in ZnO varistor ceramics.