Using Geotail data from the months of December 1994 and January 1995, we have investigated transverse fluctuations in the magnetic field and in the plasma velocity in the magnetosheath. In the frequency range below about 10 mHz, a change in the magnetic field is often linearly correlated with a change in plasma velocity, satisfying the Wal¿n relation expected from the theory of MHD wave propagation. We found well-correlated events in a wide range of radial distances of the magnetosheath, -42 RE<GSM--X<6 RE, where RE is the radius of the Earth. The correlation is sometimes positive and sometimes negative, depending upon the angle between the magnetic field and the plasma flow direction. The results indicate that, in a frame moving with the plasma, the waves propagate unidirectionally in the same direction as the plasma flow in a draping field. In other words, the wave energy is always propagating simultaneously antisunward and outward. Moreover, our results suggest two possibilities for the origin of the signals in the magnetosheath: the magnetopause or the nose region of the bow shock. ¿ 2000 American Geophysical Union