We used tangential component SH waveforms with S, ScS, sS and sScS arrivals of Chilean earthquakes recorded by four seismographic stations located in Antarctica to study the radial velocity structure at the core-mantle boundary (CMB) beneath the South Eastern Pacific basin. The selected station-event pairs give information on two distinct, close, patches of D'' beneath the Bellingshausen Sea. In one of these regions an anomalous arrival-interpreted as a reflection at the top of D'' (SdS phase)-is seen between S and ScS, together with early arrivals of ScS. This observation is well fit by adding to global reference model SP6 a reflector 315 km above the CMB, marking a 3% S-velocity increase. The new velocity model, usb2, fits both SdS and ScS travel times and amplitudes. Available data do not provide clear evidence to confirm or refute the presence of a similar velocity discontinuity in the second patch.¿ 1997 American Geophysical Union