The differential equation x + x = -A(x2 - l)x + fl kos (t) is a special case of equations of the general type x + x = h(x, x, t, fl, A). Our main aim is to derive the bifurcation equation of periodic solutions of the former equation, in the first quadrant of (X, jl) -plane. We show that the number of periodic solutions change from one to three as parameters fl and X cross the bifurcation equation fl = — A.