Smooth muscle myosin filaments formed in 0.15 M KCl are depolymerized by MgATP to a 10S component, rather than to the 6S component typical of myosin monomer in high salt concentrations. This 10S species is also monomeric as determined by sedimentation equilibrium and calculated from the diffusion and sedimentation coefficients. The conformation of 10S myosin is, however, very different from that of 6S myosin, which has a flexible but extended rod. The Stokes radius and the viscosity of 10S myosin are less than those of 6S myosin, consistent with a structure in which the rod is bent. Electron microscopy of rotary-shadowed preparations confirmed that the light meromyosin region of the rod is bent back on subfragment 2, that region of the rod adjacent to the two globular heads. MgATP and dephosphorylation of the 20,000 molecular weight light chain increase the amount of 10S myosin present in 0.15 M KCl; addition of salt converts 10S myosin back to the typical 6S conformation. We conclude that smooth muscle myosin preferentially forms a bent or folded conformation instead of the extended shape usually associated with skeletal muscle myosin, provided that the salt concentration is kept sufficiently low.