Dispersion relations for electrostatic surface waves in a homogeneous plasma-layer \vhich interacts with a n external electric pumping field parallel to the boundaries are derived in the framework of the kinetic theory. The parametrically excited and purely growing instabilities are investigated. Electric threshold fields and growth rates for the instable surface modes are given. It can be shown that the minimum threshold for parametric excitation occurs for those modes which are symmetric about the midplane of the layer.