We analyzed the seismicity of the Vesuvius volcano in order to estimate the hydraulic diffusivity &agr;, the permeability k, the pore-fluid pressure and the stress magnitude in the seismogenetic volume. Vesuvius seismicity develops within the caldera volume and an events clustering occurs at about 2 km of depth. &agr; has been estimated with two independent methods based on (a) the analysis of time history of earthquake occurrence combined with rainfall data of the last 25 years and (b) the progressive migration of earthquake foci observed during the February--April 1996 seismic sequence. The obtained value (&agr;=0.25 m2 s-1) has been utilized to calculate the permeability of the Vesuviuslavas (k=5.3⋅10-14 m2). Assuming a Coulomb criterion of failure, we performed a stress analysis of focal mechanisms in order to estimate the values of the ratio &lgr; between the pore fluid pressure the vertical stress for which the nodal planes have sufficiently resolved shear stress to slip. We found &lgr;=0.6. We conclude: (a) the caldera volume is affected by an intense hydrothermal fluid circulation; (b) the spatio-temporal features of the seismic activity are governed by increases in pore-pressure; (c) the maximum rock strength weakening occurs at the transition between the sedimentary rocks of the volcanic basement and the intra-caldera volcanics. ¿ 1999 American Geophysical Union