A numerical study is carried out on the natural convection taking place in a horizontal saturated porous layer. A sinusoidal temperature distribution, as a moving wave, is superposed on the hot temperature of the lower plate. The imposed wavelength of this perturbation is equal to the wavelength of incipient Benard cells. For a given Rayleigh number R, results show that the Benard cells move with the imposed wave if the velocity of the latter remains below a critical value. Beyond this critical value, there is still an entrainment of the cells, but at a much lower rate. Further, the cell motion is irregular and time periodic.