Phosphorylation of the 20,000-molecular weight (M_r) light chains of vertebrate non-muscle (thymus) and smooth muscle (gizzard) myosins regulates the assembly of these myosins into filaments in vitro^1,2. At physiological ionic strength and pH, nonphosphorylated smooth muscle and non-muscle myosin filaments are disassembled by stoichiometric levels of MgATP, forming species having sedimentation coefficients of ∼11S (range 10–12S^1,3,4; myosin monomers in high salt sediment at 6S). When the 20,000 (20K)-M_r light chains on these 11S myosin species are phosphorylated by the light-chain kinase/calmodulin–Ca²⁺ complex, the inhibitory effect of the light chains on filament formation is removed and the myosins reassemble into filaments which are stable in MgATP^1,2,5,6. It was originally suggested that the 11S myosin species was a dimer^1,3, previously suggested as a building block for smooth muscle and non-muscle myosin filaments^7,8. It has since been shown, however, that 11S smooth muscle myosin is monomeric^4,9 and has a folded conformation^4,10 rather than the extended shape characteristic of monomeric myosin in high salt^11,12. Here we show that 11S non-muscle myosin is also folded and that phosphorylation of the 20K-M_r light chains of both vertebrate non-muscle (thymus) and vertebrate smooth muscle (gizzard) myosins causes these folded 11S molecules to unfold into the conventional extended monomeric form, which is able to assemble into filaments.