MUSCLE contraction is driven by the cyclical interaction of myosin with actin, coupled to the breakdown of ATP. Studies of the interaction of filamentous myosin(1) and of a double-headed proteolytic fragment, heavy meromyosin (HMM)(2,3), with actin have demonstrated discrete mechanical events, arising from stochastic interaction of single myosin molecules with actin. Here we show, using an optical-tweezers transducer(2,4), that a single myosin subfragment-1 (S1), which is a single myosin head, can act as an independent generator of force and movement. Our analysis accounts for the broad distribution of displacement amplitudes observed, and indicates that the underlying : movement (working stroke) produced by a single acto-S1 interaction is similar to 4 nm, considerably shorter than previous estimates(1-3,5) but consistent with structural data(6). We measure the average force generated by S1 or HMM to be at least 1.7 pN under isometric conditions.