Significant changes in groundwater flow were observed immediately before and after an earthquake with M=6.6 occurred on March 18, 1987, in southwest Japan. The groundwater flow from a deep borehole well 226-km northeast of the epicenter had begun to increase slightly 15 minuts before the earthquake. Just after the shock, the flow rate increased rapidly and exponentially approached a constant, but there were still oscillatory fluctuations even 5 hours later. The fluctuations correspond to 0S2 and 0S3 spheroidal modes of free oscillations of the earth. The nonoscillatory transient flow must be the response to the residual field and/or to some non-recoverable deformation of the groundwater reservoir resulting from strong ground motions due to seismic waves caused by the earthquake. Fault model calculations, however, indicate that the residual strain effect on the transient flow change is unimportant. The preseismic flow increase indicates a change in regional stress field preceding the earthquake. The present observations illustrate that groundwater responds systematically to minute crustal deformations just before and after an earthquake. ¿ American Geophysical Union 1988