Studies of photochromic damage induced in KTiOPO4 single crystals by 514.5 nm laser irradiation is reported. A dependence of photochromic damage on polarization was observed. Along the same propagation direction (theta = 90degrees, psi = 23.3degrees), the gray-track susceptibility for parallel toz polarization irradiation is one order magnitude lower than that for perpendicular toz polarization irradiation for KTP crystal. The results clearly show that in KTiOPO4, a correlation exists between the gray-track susceptibility and the ionic electrical conductivity. Due to a high ionic electrical conductivity along the polar z-axis, the optical absorption centers will be annihilated with parallel toz polarization irradiation as soon as they are produced. This results in a lower gray-track susceptibility for parallel toz polarization irradiation. (C) 2002 Elsevier Science B.V. All rights reserved.