The division of chloroplasts in Trebouxia potteri was studied by electron microscopy. At the beginning of chloroplast division, vesicles with fine fibrils (FVs) and ER attach to the isthmus of the chloroplast. Then, filaments appear around the isthmus parallel to the direction of constriction and seem to contract in order to decrease the diameter of the isthmus. It is suggested that the FVs are involved in the formation of the filaments and that the ER is involved in the contraction of the filaments. At the final stages of the division of the chloroplast, the filaments decompose. FVs are partially surrounded and decomposed by lysosomal sheets. For the next cycle of division of the chloroplast, the recovery of substances from decomposed filaments by functional FVs seems a realistic possibility.