The Ontario-New York-New England refraction study (O-NYNEX) provides a set of high-quality data for a tomographic study of an intraplate continental region with low seismic activity. This study focuses on the region crossed by the United States part of the O-NYNEX profile. Tomographic inversion of the travel-time data is used to construct a three-dimensional model of the P- and S-wave seismic velocity distribution within the shallow crust of this region. Velocity structure is resolved in the top 15 km of the crust, with the most detailed picture obtained for the depth range between 1 and 5 km. The upper crust of the Proterozoic Grenville Province is found to have a P-wave velocity 0.2 to 0.3 km/sec higher than the Paleozoic Appalachians. The area of anorthositic intrusions within the Adirondack Mountains is characterized by P-wave velocities 0.3 km/sec higher and S-wave velocities 0.1 km/sec higher than the average values for the Grenville Province. This high-velocity anomaly extends to the depth over 5 km, possibly as deep as 16 km. The velocity structure of central Vermont is found to be closer to that of the Grenville Province than to the rest of the Appalachians. Central New Hampshire shows relatively small lateral variation of seismic velocity. No strong correlation is seen between velocity anomalies and intrusive bodies of the White Mountains magma series.