We have developed a 2 1/2-dimensional (x, y, vx, vu, vz) electromagnetic code to study the formation and the stability of the magnetopause current layer. This code computes the trajectories of ion and electron particles in their self-consistently generated electromagnetic field and an externally imposed two-dimensional vacuum dipolar magnetic field. The results presented here are obtained for the simulation of the solar-wind-magnetosphere interaction in the subsolar region of the equatorial plane. We observe the self-consistent establishment of a current layer resulting from both diamagnetic drift and E¿B drift due to the charge separation. The simulation results show that during the establishment of the current layer, its thickness is of the order of the hybrid gyroradius &rgr;H=(&rgr;i&rgr;e)1/2 predicted by the Ferraro-Rosenbluth model. However, diagnostics indicate that the current sheet is subject to an instability which broadens the width of the current layer. Ripples with wavelengths of the order of the ion gyroradius appear at the interface between the field and the particles. These perturbations are observed both on the electrostatic field and on the compressional component of the magnetic field. This instability has a frequency very close to the local ion cyclotron frequency and propagates with a phase velocity in the same direction as the electron diamagnetic drift. The nonlinear phase of the instability is characterized by the filamentation of the current layer which causes anomalous diffusion inside the central current sheet. ¿American Geophysical Union 1990