Propagation of shear waves produced by 25 mantle earthquakes (80--600 km depth) in the subduction zone of the south Tyrrhenian Sea (southern Italy) has been investigated to infer the geometry and extent of the descending lithosphere. From all hypocentral depths high-frequency, high-amplitude shear waves are recorded at most of the stations in southern Italy and easternmost Sicily. This shear-wave energy is interpreted to travel as a guided wave within the descending slab. In contrast, shear waves are either not recorded at all or they are recorded as low-frequency, low-amplitude signals at stations located in the peninsular part of Italy north of the Calabrian arc, in western Sicily and in Sardinia. This systematic S-wave attenuation is interpreted in terms of an active and continuous slab correlated with and limited to the Calabrian arc. ¿ 1998 American Geophysical Union