Individual bovine brain myosin molecules visualized by electron microscopy consist of two globular heads and a fibrous tail, like myosin molecules from other sources. Brain myosin, however, showed much lower solubility at moderate to high ionic strength (0.2 to 0.4 M KCl) than gizzard myosin, and the filaments formed at low ionic strength in the presence of Mg2+ were fairly resistant to low concentrations of ATP, by which gizzard myosin filaments were completely solubilized. Brain myosin was digested with low concentrations of papain, alpha-chymotrypsin, or trypsin, and the fragmentation patterns were analyzed by means of polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate, sedimentation at low ionic strength, and electron microscopy of the fragments produced. The results indicate that all of the proteases cleave the myosin molecule primarily at sites located in the neck or in the head close to the neck, suggesting that the brain myosin molecule contains a hinge region or an open peptide stretch around these sites. The differences as well as the similarities between the proteolytic fragmentation patterns of brain myosin and other myosins are discussed.