SEISMIC studies have shown that continental rifts such as Lake Baikal and the Great Rift Valley of East Africa are like midocean rifts in that they lie above broad regions of asthenospheric upwarp of much greater extent than the surface expression of rifting(1-4). The direction of mantle flow in such regions ran be investigated using the seismic anisotropy created by flow-induced orientation of mantle olivine crystals(5-8). Seismic studies of the Mid-Atlantic Ridge have revealed upwelling mantle flow beneath the ridge and flow normal to the ridge axis on either side(8-10). Here we present results from an array of seismic stations across the Baikal rift zone in southern Siberia. The splitting in arrival times of SKS seismic waves indicates that the upper mantle beneath the rift zone is anisotropic, with the fast direction (which reflects the direction of mantle how) being horizontal and normal to the rift axis. This suggests that the broad upwarp associated with this continental rift is caused by similar mantle flow to that at mid-ocean rifts. This may help to elucidate the processes involved in continental rifting.