  We study the semiclassical behaviour of a two--dimensional nonintegrable
system. In particular we analyze the question of quantum corrections to the
semiclassical quantization obtaining up to the second order of perturbation
theory an explicit analytical formula for the energy levels, which is the usual
semiclassical one plus quantum corrections. We compare the "exact" levels
obtained numerically to the semiclassical levels studying also the effects of
quantum corrections.
