In the Larderello geothermal area, reflection seismic lines show a poorly reflective upper part and a highly reflective lower part (Cameli et alii, 1993). The top of the lower part is marked by a high-amplitude reflector, named the K-horizon (Batini et alii, 1979), which has local "bright spot" features determined by fluid occurrence (Batini et alii, 1985). The K-horizon often bounds a facies with strong convergent and divergent reflections and lozenge-shaped geometry (Batini et alii, 1979). Measurements of bottom-hole temperatures, local earthquake depth distribution, and rheological estimation indicate that the depth of the K-horizon coincides with the top of the crustal brittle/ductile transition (Liotta & Ranalli, 1999), which is supposed to act a s a decoupling shear zone (Cameli et alii, 1993). Locally the K-horizon loses its strong reflectivity, and a low-amplitude reflective zone is imaged. In order to investigate the relationship between geologic structures and reflectivity of the top of the brittle/ductile transition, several seismic reflection lines were analysed by integrating seismic interpretation with borehole and field data. The results indicate that the disappearance of the strong reflectivity is related to the occurrence of three main north-east dipping roughly parallel extensional shear zones. Their surface expressions are systems of listric normal faults, affecting (from west to east) the Pliocene Serrazzano Basin, the Cornate area, and the Pliocene-Quaternary Pomarance-Radicondoli Basin. Some of these faults are mineralized by mixed sulphides of hydrothermal origin related to the Pliocene-Quaternary Tuscan magmatism. The identification of three main crustal brittle shear zones, and their link with mineralised outcropping normal faults, suggest enhancement of fluid circulation in these zones. Therefore, the disappearance of strong reflectivity of the K-horizon may be determined by the local absence of trapped fluids, which have migrated toward shallow reservoirs along the shear zones. At these shallow levels, these deep fluids are mixed with meteoric water, producing the present Larderello geothermal fluids.