Our previous titration and cross-linking experiments showed that myosin subfragment 1 (S1) can bind to one or two monomers in F-actin [Andreev, O. A., & Borejdo, J. (1991) Biochem. Biophys. Res. Commun. 177, 350–356; (1992a) J. Muscle Res. Cell Motil. 13, 523–533; (1992b) Biochem. Biophys. Res. Commun. 188, 94–101]. In the present work we used a sedimentation method to extend these studies to equilibrium binding and a stopped flow method to investigate its kinetics. Both equilibrium and kinetic data indicated the existence of two different rigor complexes. On the basis of these data we developed a model which suggested that binding of S1 to F-actin occurred in two steps: (i) initial rapid binding to one monomer of F-actin, A + M ↔ A·M and (ii) a consequent slow binding to a neighboring monomer, A·M + A ↔ A·M·A, where A stands for actin and M for myosin subfragment 1. The second reaction can proceed only if the neighboring actin site is unoccupied. The model fit the equilibrium and kinetic binding data with equilibrium constants K1 = 6 × 106 M−1 and K2 = 4 and kinetic constants k+1 = 10.5 × 106 M−1 s−1, k−1 = 1.75 s−1, k+2 = 0.8 s−1, and k−2 = 0.2 s−1, where the subscripts refer to the reactions i and ii. These results corroborate our hypothesis that myosin head can make two types of complexes with F-actin and support our speculation that during a power stroke in contracting muscle a myosin head may first bind to one and then to two actins.