Shear experiments on olivine-basalt aggregates provide compelling evidence that the dynamic distribution of melt is controlled by the magnitude and orientation of the differential stress. Our results suggest that deformed, partially molten upper mantle rocks will have highly anisotropic physical properties including seismic wave velocities and melt permeability. In addition, our results provide a basis for interpreting geophysical observations, such as shear-wave splitting and for modeling melt migration processes beneath mid-ocean ridges, specifically focused flow of melt toward the ridge axis. ¿ 1999 American Geophysical Union