We investigate the regularity of particle motion in a magnetopause-like magnetic field topology for a southward interplanetary magnetic field case. For that issue, a rotation of 180¿ of the magnetic field direction through the magnetopause is considered, while its total magnitude remains constant. Our main interest is to emphasize the different dynamical regimes for charged particles when their Larmor radii are of the order of the thickness of the magnetic field reversal layer. When the interface is a tangential discontinuity, it is shown that particle dynamics is regular, whereas the introduction of a small component of the magnetic field, normal to the interface, results in weakly stochastic dynamics. A new parameter &kgr;RL is introduced to organize the charged particle dynamics. When &kgr;RL is greater than 1, particle motion is adiabatic, and when &kgr;RL is smaller than 1, particle motion is nonadiabatic, its magnetic moment being possibly either damped or unchanged when interacting with the discontinuity. Implications for the pitch angle distribution of a population interacting with such a tangential discontinuity will be discussed. ¿ 2000 American Geophysical Union