We describe a new nonlinear-optical phenomenon called the intensity-activated birefringence zero-crossing shift. This phenomenon leads to the shift of the transmission curve of the gyrotropic isoindex spectral filter. The phenomenon was observed with 33-psec, 532-nm Nd(3+):YAG laser pulses in CuAlSe(2) semiconductor crystal. An anisotropic refractive-index nonlinearity with an approximately 200-psec switch-off time is responsible for the shift. Possible applications for light by light-control devices are discussed.