A magnetotelluric (MT) survey has been performed in Gediz Graben located in western Turkey. The main goal of the survey is to search for additional geothermal sources to enlarge the city heating system. Three-dimensional (3D) inversion of the full impedance tensor provides valuable information on the current reservoir situation and reveals that it has no extra capacity for new subscribers. However, the low resistivity anomalies in the deeper part of the graben sedimentary fillings is probably related to clay alteration zones caused by hydrothermal activities. The hot fluids outflow along the cracks and fractures of normal faults presence in the study area. Interpretation of the 3D resistivity model suggests that the extension of normal faults in the metamorphic basement can be considered as a promising reservoir zones.