Electrical conductivity measurements were made on black shale samples from deep boreholes under laboratory conditions corresponding to more than 10 km burial depth. At these conditions, large differences in resistivity were observed for shales of different diagenetic/metamorphic grade, although the total organic carbon (TOC) and sulfide content are comparable. A bituminous black shale (low diagenesis) showed reversible, temperature dependent changes of resistivity from 105 &OHgr;m at room temperature to 104 &OHgr;m at 300 ¿C and 250 MPa. These are typical values for dry crustal rocks. For a very low grade metamorphic black shale (transition from high diagenesis to beginning of metamorphism) a decrease in resistivity of at least 1 order of magnitude to a value of about 1 &OHgr;m was observed at the same p, T conditions. This type of black shale shows permanent changes in resistivity. The high conductivity achieved in this sample is mainly caused by the transition of primary pyrite to pyrrhotite and the increase in connectivity between the conducting phases produced by the more elongated geometry of the newly formed sulfides. Both factors result in an increase in conductivity at in situ conditions. These results confirm that the known parameters of TOC content, amount of primary sulfides, and coalification stage are important for determining the conductivity of black shales. But the analytical investigations of the samples after the experiments indicate that the degree of foliation and sulfide transformation at low metamorphic conditions also plays an important role. The foliation and the coalification as well as the geometrical form of the newly formed sulfides are connected with the diagenetic/metamorphic overprint of the shales. Midcrustal conditions (temperature, pressure, low oxygen fugacity) seem to promote the mobilization of sulfur and/or iron and result in increased connectivity of conducting components. Therefore some highly conducting zones in Earth's crust might be caused by a combination of organic carbon at least in a weak metamorphic stage and sulfides. ¿ 1998 American Geophysical Union