The hydrogeology of fractured crystalline rock is often controlled by a few highly conductive features such as faults and/or fractures zones. Accurate modeling of fluid flow and transport in these systems requires that the geometry of these conductive features be characterized explicitly. A new method for estimating the geometry of these features is presented. The method is an extension of the recently developed geostatistical technique of sequential cokriging. In this extension, the geometry of the fracture zone is iteratively updated until an update increases the discrepancy between the estimated and observed hydraulic heads. Results from two hypothetical examples demonstrate that the method can accurately estimate the geometry of highly conductive fracture zones. ¿ American Geophysical Union 1996