Although it is generally accepted that earthquakes occur along preexisting faults, the distribution of earthquake locations is often so smeared that the underlying fault or joint structures along which the earthquakes occur cannot be inferred from visual inspection of location plots. We present a statistical method for identifying fault of joint planes within what may otherwise appear to be an amorphous earthquake location set. The method takes all the hypocenters in an event set three at a time in order to determine the strikes and dips of all possible planes within the event set. A procedure for correcting for the shape of the region in which the earthquakes occur is applied. After correction, the orientation (one or a few) that is seen most often is taken as that of the zone of preexisting fault(s) of joint(s). We applied the method to a set of hypocenters determined for microearthquakes that accompanied a hydraulic injection into crystalline rock. The method was able to resolve successively five statistically significant orientations (planes) along which most of the microearthquakes occurred. The first two planes determined by the method are parallel to one nodal plane from each of the two most commonly found fault plane solutions. One of the two planes intersects the injection well bore at a location where water is known to have entered the rock during the injection. The planes identified thus coincide with the major fluid paths during the hydraulic injection. ¿ American Geophysical Union 1987