During forced water infiltration into a dry granite rock in the dilatant state, migration of a water front and a clear positive feedback between acoustic activity and water flow into microcracks were revealed by means of AE hypocenter locations and velocity tomography. Water was injected into a cylindrical granite sample subjected to 40 MPa confining pressure and constant axial stress equal to about 70% of short-term dry strength. The injection water pressure was held constant at 17 MPa until failure occurred. Soon after water injection into the bottom end of the sample, AE hypocenters began near the bottom end. Subsequently, they moved up with a strong preference for the region where a weak concentration of AE events had been observed during a steady state creep stage before the water injection. The P wave velocity structure was determined in the section parallel to the loading axis and crossing the cluster of AE hypocenters. The velocity structure, reconstructed by seismic tomography, showed a nonsymmetric distribution of the velocity with regard to the loading axis during the injection. The strong concentration of AE hypocenters suggests that water flow channels are preferentially formed in a very limited region in the dilatant rock. The concentration of AE events and formation of these water channels marks a positive feedback process in which the local strength of the rock is reduced by the decrease of the effective confining pressure caused by increasing pore pressure which, in turn, triggers more microfracturing. ¿ American Geophysical Union 1990