P wave arrival time data from 45 earthquakes and three explosions recorded on the 37-station Hawaii network were inverted simultaneously for hypocenter parameters, compressional wave velocity as a function of depth, and station corrections. Hypocenter depths range from near surface to approximately 35 km, with most events near Kilauea volcano. The structure model obtained has a distinct and well-constrained low-velocity zone at the base of the crust. This feature is most readily explained as a serpentinized transition layer, although it is difficult to completely rule out a thermal origin. Upper mantle velocity at the M discontinutiy is about 8.3 km/s with a gradient estimated to be about a 0.01-km/s/km increase in velocity with depth. Evidence is found for a crust which is depressed beneath the island, presumably in response to the volcanic load. Station corrections exhibit an inverse correlaton with regional Bouguer gravity variations. The summit of Mauna Loa volcano in particular has arrival times anomalously early by as much as 0.5 s compared to flank stations, indicating that a high-velocity core in the volcano extends nearly to the base of the volcanic pile.