Near the tail boundary beyond about 100 Re GEOTAIL often encounters a plasma mantle-like boundary layer in which the plasma flowing tailward transitions smoothly from magnetosheath values of speed and density to much smaller values more characteristic of the tail lobe. This boundary layer had earlier been recognized on the basis of ISEE 3 measurements. GEOTAIL confirms the existence of this layer and extends the documentation on its behavior. Boundary oscillations sweep the boundary layer over the spacecraft enabling GEOTAIL to ''sound'' the layer's profile of plasma parameters. The density-versus-speed correlogram of the mantle-like part-a useful diagnostic for comparisons--is reasonably well simulated by a 1-D, MHD slow-mode expansion fan model of the plasma mantle. Flow directions are consistent with an open plasma mantle on the northern, duskside flank of the tail, as expected for the standard magnetic merging model of mantle formation. ¿American Geophysical Union 1994