It is shown that for a medium with the dielectric function given by the expression ∈ = ∈[sub 1] + (∈[sub 2] - ∈[sub 1]tanh(E²/E²), where ∈[sub 1] and &isinl;[sub 2] are constants and E[sub s] is the saturation field, there is a threshold value of the power density at which a nonlinear surface wave can be excited.