The Alban Hills is a Quaternary volcano located along the Tyrrhenian margin of the Apennines (Central Italy), which has erupted from 0.7 to 0.027 Ma. The high density of population around the volcano as well as the proximity to Rome (∼20 km) spurred us to investigate the three-dimensional (3D) crustal structure beneath the volcano, with particular emphasis to the identification of volcano-related phenomena. Hydrothermal circulation and frequent seismic activity characterize the present state of the volcano. A seismic swarm from April 1989 to March 1990 consisted of shallow seismicity (depths less than 6 km), located beneath the western side of the volcano, where the phreatomagmatic eruptions (0.03 Ma) occurred. In this paper, we present new results obtained from seismic tomography using the recorded local earthquakes. We imaged the earthquake distribution jointly with the 3D velocity structure by inverting 1314 P and 1185 S arrival times from 163 selected events using the method developed by Thurber (1983). We progressively focused toward the well-sampled crustal volume, decreasing the model gridspacing in subsequent inversion. Detailed resolution analysis and comparison with results of a synthetic test ensure that the computed models are reliable, at least in the volume where the ray coverage is sufficient. The calculated model improves the hypocentral locations of the 1989-1990 earthquakes, allowing us to visualize the structures that generated the seismic swarm. The relocated earthquakes are confined between 3 and 6 km in depth, and they surround a body with anomalously high P-wave velocity that is probably uplifted and metamorphosed carbonate rocks.