An electron microscope study of mitochondria in hamster liver and kidney cells has revealed that at some points the outer membrane of these organelles is continuous with the inner membrane. Also, at such points the discontinuous components of the membrane pairs have free endings. The outer and the inner membranes of a mitochondrion, therefore, may not be two different and distinct entities, as has been conventionally assumed, but may rather be a part of the same unit. Such a morphological structure would make the intramitochondrial substance accessible to the cytoplasmic substance through the intermembrane channel. This structure would also facilitate the swelling of a mitochondrion either by an unfolding of the cristae, or a sliding of the two membranes, or by both these processes occurring simultaneously.