Electron microscopically, the myosin molecule from the plasmodium of Physarum polycephahum has a long tail of 173 nm, having a flexible region over the range of 80 to 120 nm from the head-tail junction. In 0.6M ammonium acetate, this region of the dephosphorylated myosin molecules is more flexible than that of the thiophosphorylated ones. In 50mM ammonium acetate, the dephosphorylated myosin molecules exist in monomeric and oligomeric forms, independently of ATP and Mg²⁺, whereas the thiophosphorylated myosin molecules form dense aggregates of thick filaments. The tails of the monomeric dephosphorylated myosin molecules bend sharply at the flexible region at angles of more than 120°. In oligomers of the dephosphorylated myosin molecules, the molecules are all associated sideto-side with straight tails and are oriented in the same direction. Based on these results, the regulation mechanism of cell motility of the plasmodium is discussed.