REGIONS of the Earth’s upper mantle show significant seismic anisotropy due to the preferred crystallographic orientation (’lattice preferred orientation’) adopted by its constituent minerals in response to deformation(1-6). Seismic anisotropies thus provide clues to the flow and/or stress patterns in the upper mantle, but the use of lattice preferred orientation to infer such properties from seismic data has been hampered by the lack of experimental studies relating changes in crystallographic orientation to simple-shear deformation. (Simple shear is probably the dominant mode of deformation in the upper mantle, whereas most previous experiments have focused on the effects of uniaxial compression(7-9).) Here we describe the results of simple-shear deformation experiments on olivine aggregates, conducted at high temperatures and pressures (similar to 1,500 K and 300 MPa). For large strains (up to 150%), our experiments reproduce the lattice preferred orientation observed in highly deformed upper-mantle rocks, in which the olivine [100] axes lie nearly parallel to the bow direction. But for relatively small strains, the preferred orientation is rotated with respect to the flow direction, indicating that seismic anisotropy should also be sensitive to the sense of shear is tbe upper mantle.