In the presence of excess amounts of fluorine, a physiological divalent cation, magnesium (Mg²⁺), forms a novel phosphate analogue, magnesium fluoride (MgFn). Park et al. [Biochim. Biophys. Acta 1430, 127-140 (1999)] previously demonstrated that MgADP•MgFn forms a complex with myosin subfragment-1 (S-1), and the S-1, ADP•gFn ternary complex mimics a transient state in the activity cycle of ATPase. In the present study, localized conformations in the regions of highly reactive cysteine and lysine residues, Cys 707 (SH1), Cys 697 (SH2), and Lys 83 (RLR), which change their conformations markedly during ATP hydrolysis, were studied using fluorescent probes and chemical modification. The global shape of the complex was also studied using small angle X-ray solution scattering and compared it with other previously reported myosin-ADP•fluorometal ternary complexes. The results suggest that the overall conformation and localized functional regions of the complex are quite similar to those in the presence of ATP, indicating that the complex mimics the M¨•ADP•P_i steady state.