A shear-wave generator has been constructed to produce shear waves for site-response and attenuation studies. The driving force of the generator comes from a double-acting air cylinder. When in use, the device is firmly held on the ground surface by the weight of the front end of a truck. Two hammers, attached to the ends of the air-cylinder piston rod and sliding on low-friction tracks, alternately impact anvils clamped between two aluminum channels. The resulting traction exerted on the ground surface by the lower channel produces a repeatable wave field having a large shear-to-compression amplitude ratio in directions perpendicular to and bisecting the channel. Signals generated by the device are highly repeatable. Using low-noise waveforms recorded close to the source as a timing reference, waveforms recorded farther away have been stacked to enhance the signal-to-noise ratio, indicating that the new device can be used for shear-wave arrival time and amplitude measurements over extended distances.