We have analyzed 42 microearthquakes (1<ML<3) that occurred at Campi Flegrei volcanic area during an intense swarm-like activity (more than 500 events in half a day) in the last period of strong ground uplift (1982--1984). Focal mechanisms of these earthquakes have been computed by a probability method, which uses P wave polarity and S wave polarization data. Composite focal mechanisms for different groups of earthquakes have also been computed by the same technique. Results show a well-defined fault plane, in agreement with the elongation of the epicentral pattern. The orientation and dip of this fault plane, together with other observations about hypocenter locations and focal mechanisms of the whole seismicity accompanying the ground uplift episode, suggest the presence of an elliptical fracture system at the caldera center. This fracture system, dipping toward the center of the caldera, should represent the-limit of the most recent caldera collapse, which is also indicated by a negative Bouguer gravity anomaly. Moreover, an important differential feature among various groups of earthquakes has been shown, namely, a rotation up to about 30¿ of the strike of the compressive stress axis, between the easternmost and the westernmost events. The resulting pattern indicates that the axis is always oriented towards the town of Pozzuoli, which roughly represents the very center of the caldera, and also the center of symmetry of the ground deformation. These observations strongly suggest that earthquakes are generated by the variation of the stress field associated with the deformation, along prefractured zones, because of their greater weakness. This study provides, then, the first direct evidence that both seismicity and ground deformations have a common origin. ¿ American Geophysical Union 1995