The ultrastructure of cell division, with the emphasis on septum formation, is described in the ulvophycean green alga Ulothrix palusalsa Lokhorst. Septum formation involves a cleavage furrow which is initiated before the onset of mitosis. The development of the cleavage furrow is most pronounced at the final mitotic stages when it shows a serial arrangement of alternating thickened and flattened parts. Especially in the chloroplast region, a hoop of microtubules is sometimes associated with the leading edge of the cleavage furrow.