A three-dimensional compressible resistive magnetohydrodynamic simulation code, with inclusion of the fully generalized Ohm's law, has been developed to study the nonlinear evolution of field line resonances in Earth's magnetosphere. A simple Cartesian box model of an inhomogeneous plasma with straight geomagnetic field lines is used, and the Alfv¿n velocity increases monotonically from the magnetopause boundary layer toward Earth. A monochromatic fast mode compressional oblique wave is applied from the direction of the magnetopause boundary layer, pumping energy into the magnetosphere. The fast mode wave, while propagating toward Earth, is partially reflected at the turning point (located at radial distances between 8 and 10 RE in the equatorial plane) and then couples to shear Alfv¿n waves, leading to the formation of large-amplitude field line resonances near Earth. The field line resonances are observed to narrow to several electron inertia lengths within several wave periods of the driver wave, and electron inertial effects become important at this stage. Final profiles near the resonance are very similar to Airy functions, indicating that electron inertial effects become important before possible nonlinear effects. The electron inertial effects lead to oscillating parallel electric field which might be potential accelerators for electrons in some types of auroral arcs. ¿ American Geophysical Union 1994