In this paper we study the topological entropy of the motion of a particle of charge on a closed manifold M under the effect of a magnetic field. The function is an even function and we have shown in [6] that the considered system is not Anosov if is sufficiently large. Let be a maximal interval such that the motion of the particle of charge is Anosov. We show that strictly decreases on . It follows that is an absolute maximum of h on the interval . We also obtain upper estimates for the first derivative of h involving the magnetic field and the curvature of M.