A model for the 3D P- and S-wave velocity structure at Parkfield has been developed by the systematic inversion of arrival times for microearthquakes and surface Vibroseis sources, recorded by nearby CALNET stations and by the local borehole-installed 10-station High-Resolution Seismic Network there. A modification of the linear B-splines model parameterization method of Thurber (1983) to a cubic B-splines formulation, through tests inverting synthetic data sets for the Parkfield configuration, is found to produce smooth velocity distributions that still retain some small-scale features. Application of the method to 953 P-wave and 288 S-wave arrivals from 111 microearthquakes and 8 Vibroseis locations throughout the study area yields a velocity structure having variable resolution. A low-velocity zone appears to coincide with the creeping segment of the San Andreas fault zone northwest of the 1966 Parkfield earthquake epicenter. An increase of about 15 per cent in seismic velocities is seen from the northeast to southwest across this segment of the fault zone. The higher velocities to the southwest apparently correspond to the granitic basement. The contrast across the fault is subdued to the southeast along the 1966 rupture zone. A high Vp/Vs ratio of 1.9 is seen in a 2-km-wide volume near the location of the 1966 main shock hypocenter, presumably the nucleation zone for the expected M = 6 Parkfield earthquake.