The behavior of cell nuclei, mitochondria and chloroplasts was followed during the mitotic cycle in the primitive unicellular red alga Cyanidioschyzon merolae by epifluorescence microscopy after staining with 4',6-diamidino-2-phenylindole (DAPI) or 3,3'-dihexyloxacarbocyanine iodide (DIOC6), by fluorimetry using a video-intensified microscope photon-counting system (VIMPCS), and by electron microscopy. The cells were classified into five types based on differences in the shape, size, and distribution of organelles in each cell: type I, II, III, IV, and V. Types II, III, IV, and V appear in that order concomitant with a decrease in type I after the cells are subjected to darkness during synchronous culture, suggesting that the organelles divide in the following sequence: chloroplast, mitochondrion, and cell nucleus. The mitochondrial DNA (mt-DNA) and the chloroplast DNA (cp-DNA) are synthesized during stage I, while the cell-nuclear DNA (cn-DNA) is duplicated in stage II. The mitochondria- and chloroplast-nuclear divisions are completed in stage IV. The mitochondrial and chloroplast divisions begin simultaneously in stage II, and chloroplast division finishes just prior to mitochondrial division. The end result is that C. merolae multiplies by binary fission. Electron microscopic examination of serial thin sections of the chloroplast clearly show that the chloroplasts in types II and III are divided by a chloroplast dividing ring (CD-ring), which is located on the cytoplasmic side of the outer envelope at the equatorial region between the daughter chloroplasts. The CD-ring may be involved in chloroplast division throughout the plant kingdom in primitive unicellular algae and higher plants.