Seismic experiments conducted on ice streams B, part of the marine ice sheet of West Antarctica, show a meters-thick layer immediately beneath the 1000-m-thick ice. A seismic experiment consisting of wide-angle reflection profiling along a line parallel to ice stream flow was conducted to determine the properties of this layer. Inversion of seismic travel times yields a compressional wave speed of less than 1700 m s-1 and a shear wave speed less than 160 m s-1 for the layer. These very low wave speeds imply that the material in the layer is highly porous and is saturated with water at a high pore pressure. Based on wave speeds in other saturated, unconsolidated sediments, we believe that a porosity substantially greater than 0.32, probably around 0.4, and an excess of overburden pressure over pore pressure of only 50 kPa (0.5 bar) characterize the layer at this location. ¿ American Geophysical Union 1987