The Campi Flegrei caldera, located near the city of Naples, Italy, has shown signs of unrest, characterized by large ground deformation, seismic activity, and changes in the hydrothermal system since 1970. Any attempt to model the processes giving rise to these phenomena requires some knowledge of the subsurface structure and of the physical properties of the rocks at depth. In this study we report the results of laboratory measurements of ultrasonic compressional and shear wave velocities in rock samples cored in 4 different geothermal wells reaching a maximum depth of 3 km. The wells are located in the San Vito (SV) and Mofete (MF) areas, at about 3 km north and 4 km west, respectively, from the center of the caldera. We selected 10 core samples, 7 from the SV wells and 3 from the MF wells, the attention being focused on the most important rock formations underneath Campi Flegrei and on the lower sections of the wells where data had been lacking. The measurements involved the transmission of ultrasonic waves along three orthogonal directions through dry and water-saturated rocks at room conditions. The samples are only moderately an isotropic (13% for one SV sample and <8% for all the other samples). Wave velocities generally increase with the depth of the sample. The range of velocities is 3--5.4 km s-1 for P waves and 1.5--2.9 km s-1 for S waves. For the SV samples, water saturation generally implies slightly larger (<10%) VP values and smaller (<20%) VS values as compared with ''dry'' velocities. For the MF samples, water saturation implies both higher VP and VS velocities (up to 35% and 18%, respectively). These results may be explained by the different type of porosity of the SV samples (vesicular) and MF samples (fissural). The ultrasonic measurements on water-saturated samples are slightly higher than the available in situ VP sonic measurements (25% for one SV sample, and <15% for all the other samples). These differences can be explained by velocity dispersion between ultrasonic and sonic frequencies and by the different pressure--temperature conditions existing at depth. ¿ American Geophysical Union 1994