Smooth muscle cells maintain filaments of actin and myosin in the presence of ATP although dephosphorylated myosin filaments and actin-myosin interactions are unstable under those conditions in vitro. Several proteins have been identified that stabilize myosin filaments and that stabilize actin-myosin interactions. Fesselin or synaptopodin 2 appears to be another such protein. Rapid kinetic measurements and electron microscopy demonstrated that fesselin, isolated from turkey gizzard muscle, reduced the rate of dissociation of myosin filaments. Addition of fesselin increased both the length and thickness of myosin filaments. The rate of detachment of myosin, but not HMM, from actin was also greatly reduced by fesselin. Data from this study suggest that fesselin stabilizes myosin filaments and tethers myosin to actin. These results support the view that one role of fesselin is to organize contractile units of myosin and actin.