The velocity profile at the base of the liquid core is derived from the mean travel time of the PKP(BC+Cdiff) data collected by the International Seismological Centre. PKP(BC) corresponds to rays having their turning points in the lower third of the liquid core, which give at great distance a branch Cdiff diffracted along the inner core boundary. Travel time residuals are computed with respect to the Earth model PREM (Dziewonski and Anderson, 1981), whose liquid core nearly follows the Adams-Williamson equation at any depth, implying a neutral density stratification. Positive values of PKP(BC+Cdiff) residuals are obtained for epicentral distances greater than 151¿. They are well explained by a liquid core model in which the velocity is constant in the lowermost 150 km. This strong decrease of the velocity gradient is in favor of a stable stratification at the base of the liquid core. It cannot be explained by thermal effects only but may correspond to a departure from homogeneity, probably in relation to the crystallization of the inner core. ¿American Geophysical Union 1991