Electrical resistivity and effective porosity have been measured for 52 drillcore samples representing major sedimentary and igneous lithologies exposed in the Sudbury Structure (SS). The purpose was to constrain interpretation of high conductivity anomalies observed by surface electromagnetic (EM) surveys conducted across the SS, and to provide information on its origin. While zones of low resistivity are known to exist in the upper sedimentary formations, rocks at depth are generally characterized by high resistivities (104-7¿104 &OHgr;-m), leaving the source of the deep EM anomaly unexplained. However, samples with low resistivities (7--20 &OHgr;-m) have been unexpectedly found in the cores from lower sections of the SS, depths possibly corresponding to that of the EM anomaly. These samples are characterized by low porosities (0.3--0.6%), normal bulk densities (2.78--2.81), and a porphyritic texture with a blackish matrix, suggesting carbon as the likely cause of the low resistivities. ¿ American Geophysical Union 1994