The first magnetotelluric study in the Marmara Sea, Turkey, was undertaken to resolve the structure of the crust and upper mantle in the region, and to determine the location of the westward extension of the North Anatolian Fault (NAF) in the Cinarcik area. Long-period ocean bottom magnetotelluric data were acquired at six sites along two profiles crossing the Cinarcik Basin, where a significant increase in microseismic activity was observed following the devastating 1999 Izmit and Duzce earthquakes. 2-D resistivity models indicate the existence of a conductor at a depth of similar to 10 km in the middle of both profiles along with a deeper extension into the upper mantle, implying the presence of fluid in the crust and partial melting in the upper mantle. The northern and southern boundaries of this conductor are interpreted to represent the northern and southern branches of the NAF in the Marmara Sea, respectively. These conductors have been previously identified farther to the east along the NAF, suggesting that the electrical characteristics of this fault are continuous from onland areas into the Marmara Sea. Microseismic activity in the Cinarcik area is located above the conductor documented here, and indicates a possible seismogenic role of crustal fluids present in the conductive zone. In comparison, resistive zones along the NAF may act as asperities that could eventually result in a large earthquake.