A 5 MHz Quartz Crystal Microbalance was used to investigate changes in resonant frequency and motional resistance of the protein film during in vitro actomyosin motility on a poly (tert-butyl methacrylate) surface. QCM crystal frequency was found to decrease with adsorption of heavy meromyosin (HMM) to the crystal surface, and with binding of additional protein during the standard BSA blocking step. The frequency and resistance signals after binding of dead HMM heads in actin rigor complexes were consistent with those expected for a film becoming more rigid, but suggested that little mass was added during this step. Addiion of the low concentration of actin used for motility did not cause a significant signal response, but addition of ATP to initiate actin filament movement caused both frequency and resistance signals to increase slightly, consistent with a less rigid protein film of lower apparent mass, suggesting that moving filaments are felt with a lower effective mass than strongly bound static filaments. The frequency signal also fluctuated significantly during motility, consistent with a dynamic process occurring on the crystal surface.