Wave scattcring from a periodic interface separating two anisotropic layers in a thick elastic plate is studied in the two-dimensional case. The problem is solved by replacing the exact boundary conditions, i.e. continuous displacement and traction on the wavy interface, by approximate first order conditions on a flat reference surface. Numerical results are presented for a number of cases and compared to the exact solution obtained by the null field approach. The conclusion is that the approximate method gives reasonably accurate results as long as the slope of the surface is small and the amplitude of the wavy surface is not too large compared to the wavelength of the incident wave.