We investigate the effect of the terrestrial magnetosphere on the dynamics of cosmic dust. We show that the magnetosphere acts as a shield with an efficiency that depends on the size and the velocity of the incoming dust particles. We report on numerical simulations for dust grains of interplanetary, cometary, and lunar origin. We show that magnetospheric effects reduce the flux of interplanetary and lunar dust particles with radii a≤0.1 &mgr;m, but the shielding is much less effective for cometary dust grains owing to their high average approach velocity. ¿ 1999 American Geophysical Union