1. A cytochrome oxidase-rich submitochondrial membrane (green membrane) was obtained from beef heart mitochondria after extraction of flavoproteins, cytochrome b, Cll C, etc. by treating with deoxycholate and potassium chloride. 2. The green membrane was formed by self assembly from the membrane fragments (flat sheets), which derived from the cristae membrane of mitochondria and had essentially the same particulate structure as the green membrane. 3. The green membrane exhibited regular arrays of small particles on the surface, measuring approximately 50 to 60 A in diameter with center to center distance of about 70 A. These particles sometime were arranged in a woven structure on the surface. 4. Both the configuration of the particles and the regularity of the arrangement were influenced by detergents and temperature. 5. Green membranes as well as beef heart mitochondria and electron transfer particles commonly retained membrane-structure after sonication and exhibited higher specific activity of cytochrome oxidase than that of purified cytochrome oxidase, if the activity is calculated on the basis of heme a concentration (sec1 / 10 m,lJ.moles of heme a/3 ml). The results suggest that the active sites of cytochrome oxidase are arranged on the surface of these membranes. 6. From these results and other experimental findings, an intimate correlation between cytochrome oxidase and the particles observed on the green membranes is suggested.