The Siljan impact structure located in central Sweden has been the object of intense geophysical and geological studies for several decades. When The Swedish State Power Board decided to drill a deep borehole in the area, investigations increased dramatically around and in the hole. In this paper we present the results of electrical resistivity measurement on the surface, in the hole, and on core samples from shallow coreholes distributed over the whole impact structure. Magnetotelluric data constrain the central part of the structure to be essentially one-dimensional with an upper crustal unit of about 10 000 &OHgr;m followed by a less resistive impact related unit of 1000 &OHgr;m from a depth of about 6 km to a depth of 20 km. The lower crust is a homogeneous unit of about 300 &OHgr;m extending down to about 40 km, where the upper mantle is marked by an increase in resistivity of about 1000 &OHgr;m. The borehole resistivity log extends down to about 6100 m, i.e., the expected depth where the resistivity drops by an order of magnitude. Major fracture zones, some of which coincide with the horizontal dolerite sills, known to extend over tens of kilometers, are probably the main carriers of current. Fracture zones are typically 10 m thick surrounded by more intact rock masses, which show a general tendency of increasing resistivity with depth. In fracture zones the electrical resistivity lies between 1000 and 5000 &OHgr;m, with the interesting trend that below 4000 m depth the resistivity is smaller than in the fracture zone in the depth interval 2000--4000 m. Saline fluids recovered during testing below 5.4-km depth and the magnetotelluric results lead us to conclude that pore fluids are highly saline below this depth even on a regional scale, and the zone of decreased resistivity found in the upper crust within the Siljan ring structure is interpreted as a zone of increased porosity relative to outside the ring. ¿ American Geophysical Union 1992