Large values of the exponential gain coefficient &Ggr; are obtained (&Ggr;&bartil;8–12 cm−1) when recording with a moving grating in photorefractive BSO crystals (nearly degenerate two‐wave mixing; drift recording mode). The resolution of the Kukhtarev’s equations with a moving grating shows a resonance effect which at the optimum velocity makes the modulation of the photoinduced space charge fieldEschigher. An optimum of the grating spacing also exists: &Lgr;opt:2&pgr;(E0/NA)(&mgr;&egr;/e&ggr;R)1/2. In such conditions, the space charge field is phase shifted by &pgr;/2 with respect to the incident fringe pattern; this allows an efficient beam coupling between the two recording beams. The dependence of the gain &Ggr; versus the incident beam ratio &bgr; of the two interfering waves is interpreted by including the second‐order term in the Fourier development ofEsc. The conditions allowing one to obtain a reasonable agreement between the theory and experiments are presented and discussed, as well as the adopted values of the crystals’ parameters.