To assess the effects of changing fluid pressure in a potential hot dry rock reservoir, two crosshole seismic surveys were conducted: one at ambient pressures and one with the investigated volume hydraulically overpressured by ~3.3 MPa. Sources and receivers were located in two straight subvertical boreholes ~100 m apart. Traveltime data were inverted for velocity information in two phases. During the first phase the seismic velocities were constrained to be isotropic, and during the second they were allowed to be anisotropic. Accounting for anisotropy enhanced greatly the interpretability of the tomograms. Before pressurization, a low-velocity anomaly at 455--475 m depth correlated with a subhorizontal zone of hydraulically permeable fractures connecting the two boreholes. Although the absolute magnitude of anisotropy is not well constrained, the dominant orientation of the TI-axes is consistent with the local stress field. Hydraulic over-pressurization resulted in a pronounced extension of the low-velocity zone, but had no significant effects on the velocity structure elsewhere. The results of this study suggest that anisotropic seismic tomography may be a key method for characterizing hot dry rock reservoirs. ¿ 1998 American Geophysical Union