THE unbinding and rebinding of motor proteins and their substrate filaments are the main components of sliding movement(1). We have measured the unbinding force between an actin filament and a single motor molecule of muscle, myosin, in the absence of ATP, by pulling the filament with optical tweezers(2). The unbinding force could be measured repeatedly on the same molecule, and was independent of the number of measurements and the direction of the imposed loads within a range of +/-90 degrees. The average unbinding force was 9.2 +/- 4.4 pN, only a few times larger than the sliding force(3-5) but an order of magnitude smaller than other intermolecular forces(6,7). From its kinetics(8) we suggest that unbinding occurs sequentially at the molecular interface, which is an inherent property of motor molecules.