Abstract Two components were present in myosin preparations from the early stage of chick embryo. The sedimentation coefficient ( s 20, w 0) were 3.4 and 6.7, respectively. The two components were separated with ammonium sulfate. Both components had ATPase activity of myosin type as rabbit myosin. The specific ATPase activity of the 6S component was higher than that of the 3S component but lower than that of adult myosin. Urea, 1–2 m, increased the ATPase activity of the 3S component but inhibited that of the 6S component. The interaction between the two components of embryonic myosin and adult actin were observed to occur at 0.6 m KCl as well as at lower ionic strength. Monomeric particles of each component were observed under an electron microscope by a shadow casting technique; their gross shape resembled that of rabbit myosin. Organized filaments of each component were formed at low ionic strength, only in the presence of ATP, but their shapes differed from each other. In the presence of 5 m guanidine-HCl both components of embryonic myosin showed a single sedimentation boundary. Based upon these observations a possible relationship between the two components of embryonic myosin is discussed.