The binding of fluorescently labeled heavy meromyosin (HMM) and heavy meromyosin subfragment-1 (S-l) to thin filaments of myofibrils and of rabbit psoas muscle fibers was measured under conditions of rigor and contraction. The fragments diffused rapidly into the myofibrillar space and bound specifically to the thin filaments. The fragments bound strongest and in a uniform fashion to myofibrils in which the competition from indigenous myosin was abolished by removing it with Hasselbach-Schneider solution. Under these conditions, the rigor Kavalues for HMM and S-l were 1.5 X 106M-1and 4.8 x 104M-1, respectively. The stoichiometry of binding was measured by independently estimating the concentration of actin sites. S-l was found to be capable of saturating all available actin sites in a myofibril or a fiber, but HMM could only occupy 50% of the sites.