During a 3-week period in November-December 1984, nine ocean bottom seismograph (OBS) stations were operating in the Lau basin concurrently with a land-based station in the Tongan island of Vava'u (VAV). Data from two intermediate-depth (200 km 400 km) earthquakes in the Tonga Benioff zone allowed the P wave velocity contrast between the subducting slab and the overlying mantle beneath the Lau basin to be determined. Based on relative residuals OBS-VAV and on three-dimensional ray tracing, a P wave velocity contrast of 7% is inferred. This is an average constrast for depths above 40 km. The standard error of the estimates is of the order of 2%. The data are consistent with a hot upper-mantle beneath the Lau basin where temperatures are close to the solidus temperature, but they do not necessarily require the presence of partial melt in the asthenosphere beneath the Lau basin. ¿American Geophysical Union 1991