We have used ISEE 1 and 2 data to analyse a tranverse magnetic disturbance in the afternoon sector of the Earth's magnetosheath. The disturbance, which evolved over a five minute period, had the appearance of a linearly polarised Alfven wave. Its most notable feature was the high level of spatial coherence in the signal at ISEE 1 and 2 which were 890 km apart. Points of common phase arrive at the two spacecraft with a clearly discernible time difference which we determined using a cross-spectral technique. These estimates are compared to the predicted behavior of MHD waves with similar polarizations to infer probable wave directions. We find that the observed time delays are consistent with propagation vectors having a substantial component across the ambient field and along the inward shock normal. The waves appear to originate in the vicinity of the quasi-perpendicular bow shock. The inferred direction of wave propagation has important consequences for the transmission of MHD waves from the bow shock to the magnetopause and places significant constraints on the nature of the wave source. We outline possible conditions which may lead to waves propagating along the shock normal and across the field. ¿American Geophysical Union 1990