The process of two-beam holographic coupling in optically anisotropic media is analyzed. We consider holographic media possessing birefringence, optically, and spatial modulation of these effects. In this paper energy transfer between two beams interfering in an electro-optic is analyzed as a function of the grating period of the recording hologram. The theory for two-beam coupling in Bi12SiO20(BSO) with an external electric field and a moving grating is developed. We report high coefficients in a wide range of low spatial frequencies. Exponential gain coefficients of Γ = 7 cm-1 have been reached for high intensity beams ratio and moving grating. These results have been compared with GaAs crystals.