The first identification of a mitochondria-dividing ring (MD-ring), which is located in the cytoplasm near the outer envelope membrane at the constricted isthmus of dividing mitochondria in the red algaCyanidioschyzon merolae, is reported. The MD-ring is about 50 nm wide and 10 nm thick at early stage of mitochondrial constriction and is a somewhat electron-dense circular bundle. The MD-ring is believed to be essential for the division of mitochondrion (mitochondriokinesis) since the ring appears at the equatorial region of the mitochondria just before the initiation of mitochondrial division and can be observed throughout mitochondrial division. The MD-ring has features comparable to that of the plastid-dividing (PD) ring.