A fast compressional motion of the magnetopause resulting from the interaction of an interplanetary shock and the Earth's magnetosphere is discussed. The ISEE-1 and 2 satellites were in the frontside magnetosphere before the shock. A magnetosonic wave front, the magnetopause, and the bow shock passed them in a very short time. By a combination of electric and magnetic field data it is possible to determine the magnetosonic and the magnetopause velocity. -from STAR, 23(14), 1985