An experimental investigation-was carried out in the Rice University wavetank facility with the object of determining the magnitude and variation in time of inertial forces due to wave action on large diameter pilings of circular cross-section. Mass coefficients were computed for various wave periods and wave heights using two different wave theories and comparisons are made between the results. The measured wave force and predicted wave force for a second order Stokes wave approximation are both plotted versus time. All of the Waves studied were "deep-water" waves . A discussion of the deviations of the experimental results from theory is included. A detailed description is provided of the equipment in the Rice University wavetank facility, including the "L"-shaped wavetank. The equipment performance is discussed.