An investigation of birefringence in crystals photo-induced by polarized second harmonic radiation of a YAG:Nd laser is presented. Considerable birefringence is observed even in the frequency range where there is no absorption. The interpretation of this effect based on the selective photoionization of centres by polarized pumping laser radiation is proposed. This birefringence should be taken into consideration in the analysis of the operation of lasers and other optical devices containing axisymmetric colour centres.