Mitosis and cytokinesis in Tetraedron are described. Persistent centrioles replicate before division and the pairs separate to define the future poles of the spindle whilst increasing numbers of microtubules become associated with them. By prophase, the centrioles and most extranuclear microtubules have become enclosed within a 'perinuclear envelope' of endoplasmic reticulum. The nuclear envelope near the centrioles then becomes indented and finally ruptures to form polar fenestrae during prometaphase; the extranuclear microtubules soon vanish and appear to move through the fenestrae into the forming spindle. Metaphase, anaphase, and telophase follow as usual. After mitosis, arrays of 'phycoplast' microtubules proliferate between nuclei. The cytoplasm is cleaved by membrane furrows coplanar with and growing through the phycoplast tubules. However, this cleavage is delayed until the cells have become multinucleate, and it appears to be irregular in extent and disposition in the cell until after a final set of synchronous mitoses. Then cytokinesis cuts up the cytoplasm into numerous small autospores which secrete their own wall; they are later released following rupture of the parental wall. Some autospores are binucleate which indicates that this cleavage apparatus does not necessarily cut up all the cytoplasm into uninucleate segments. Vegetative reproduction in these organisms is compared to that of other members of the Chlorococcales.