The Siljan impact structure located in central Sweden has been the object of intense geophysical and geological studies for several decades. This paper presents the results of electrical resistivity measurements 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 Ωm followed by a less resistive impact related unit of 1000 Ωm from a depth of about 6 km to a depth of 20 km. The lower crust is a homogeneous unit of about 300 Ωm extending down to about 40 km, where the upper mantle is marked by an increase in resistivity of about 1000 Ωm. 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. Saline fluids recovered 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.