Summary: The Cyanidiophyceae species Cyanidium caldarium and Cyanidioschyzon merolae have played important roles in showing the division mechanisms of mitoch ondria and plastids. The apparatus regulating mitochondrial and plastid division was formerly unkonwn. We first identified the division apparatus of plastids, called the plastid-dividing ring(PD ring), in C. caldarium and the division apparatus of mitochondria, called the mitochondrion-dividing ring(MD ring), in C. merolae. Eukaryotic cell division is therefore controlled by at least three dividing apparati---a contractile ring, an MD ring, and a PD ring---while bacterial division is controlled by a single bacterial contractile FtsZ ring.