Phosphorylation of the regulatory light chain of smooth muscle myosin efficiently regulates the actin-activated ATPase activity of myosin filaments in solution and actin movement in an in vitro motility assay, independently of thin-filament regulatory proteins. Filaments containing both phosphorylated and dephosphorylated heads move actin at intermediate rates, depending on the relative proportions of the two myosin species. The decrease in velocity can be accounted for by mechanical interactions between phosphorylated heads and 'weak-binding' dephosphorylated crossbridges. These results imply that shortening velocity could be modulated in any muscle by varying the relative proportions of two populations of crossbridges with different cycling rates.