A five-span, four-hundred-foot long, reinforced concrete box girder birdge supported on single column piers that are pile-founded was subjected to extensive dynamic tests. The pullback and quick-release method of excitation was used. Two D-8 crawler tractors were used to apply total release loads that had a magnitude of about one-quarter of the earthquke design loads. The transverse mode shapes and natural frequencies obtained from these tests were used in conjunction with the system identification procedure outlined herein. By using these techniques, it is possible to identify the in situ dynamic stiffnesses of the pier foundations as well as the abutment structures. In addition, the dynamic bending and shear stiffness factors for the superstructure deck were also obtained. Thus, carefully conducted dynamic tests of full-scale highway bridges provide detailed information about the structure. At the same time, they shed much experimental light on the soil-structure interaction aspects of the problem, which in many cases greatly affects the distribution of seismic loads in bridges.