Vibration tests were conducted on both the prototype and a 1:24-scale model of the North Fork Dam, a double curvature arch dam, to determine natural frequencies, mode shapes, damping ratios, and hydrodynamic pressures. Two vibrators mounted on the crest of the dam were used as input excitation sources for both series of tests. Electromagnetic shakers capable of a 40-lb output were used in the model tests, while counterrotating, eccentric mass exciters capable of a 5000-lb output were employed for the prototype. Velocities were measured along the crest and downstream face of the model, whereas accelerations were measured in the same locations on the prototype. Measurements in both curves were taken at the dam-reservoir interface while the structures were excited at resonant frequencies. Damping in both model and prototype ranged from approximately 2 to 5 percent of critical. These values are consistent with structural damping values for these types of structures.