Chierchia L. Non-degenerate "Arnold diffusion" (114K, LaTeX) ABSTRACT. The Hamiltonian $H\=\su{2} |I|^2 + \e V(\f;\e)$ with $V\=\cos \f_4 + \e^8 \sum_{j=1}^3 \cos(\f_j+\f_4)$ and $I\in\real^4$, is shown to exhibit ``Arnold diffusion" (or, more precisely, ``Arnold instability") on each energy surface, providing the first example of ``Arnold diffusion" for one--parameter perturbations of non--degenerate (``steep"), completely integrable Hamiltonian systems.