The process of nuclear division in a coenocytic, marine green alga, Valonia ventricosa J. Agardh, has been examined in the electron microscope. The mitotic (or meiotic) spindle is centric and totally closed throughout the nuclear division. Kinetochores are evident on the chromosomes. Spindle microtubules converge on the centrioles located to one side of the hooked polar regions of the nucleus, but distinct polar openings are absent. The spindle is conspicuously asymmetric at metaphase and is transformed to a usual form at anaphase. Separation of chromosomes, resulting from spindle elongation and shortening of the chromosome-to-pole distance, is not distinctly synchronous. At telophase the spindle assumes a dumbbell shape and abscission of the interzonal spindle seems to be accomplished by a twist of the daughter nuclei. Microtubules which are expected to participate in orientation of the cytoplasmic net-work were not observed in the cytoplasm. Cytokinesis directly leading to partition of motile cells was not observed in the present material.