Development of new processing techniques has enabled a light-and electron-microscopic study of cell division in Oedoyonium to be made; many unusual or unique features are described. A series oflight micrographs confirms the sequence of events described by earlier workers. Splitting of the cell wall at the ring has been observed in vivo; the rupture is often quite violent, and subsequent elongation is rapid for a short while. At the ultrastructural level, the premitotic movement of the spherical nucleus towards the wall ring coincides with the appearance of some nearby micro-tubules. The preprophase nucleus enlarges and becomes flattened and spindle-shaped as a sheath of microtubules envelopes it. The nuclear membrane at each pole is very drawn out, appearing as two closely apposed membranes undulating into the cytoplasm; between the membranes is a dense amorphous material, and around them are longitudinally oriented microtubules. During prophase, chromatin condensation accompanies nucleolar dispersion. Microtubules start appearing within the nuclear membrane which remains essentially intact throughout subsequent division. At prometaphase, intranuclear microtubules increase and some are attached to diffuse scattered kinetochores. By metaphase, many bundles of microtubules are attached to paired kinetochores, which have a complex, layered structure. Separated kinetochores are visible at anaphase. At telophase, the spindle becomes very elongated, and many interzonal microtubules appear. A granular and heavily staining "midbody" is found between the nuclei, and this later disperses. The daughter nuclei then come very close together, and between them is found a complex of vesicles and microtubules. A septum is then formed across the cell, evidently by these microtubules (accompanied by the vesicles) pushing out the tonoplast across the vacuole till partitioning is accomplished. The nuclei then separate prior to wall splitting. The ring formed at one end of the cell has a highly characteristic structure with two lips of toughened material at its outer edge attached to the outer wall; splitting of the older wall occurs between these lips. A proliferation oflarge, electron-transparent golgi vesicles was apparent at this stage; these were apparently being discharged into the vacuole, and so may have been associated with a possible build-up of turgor pressure within the cells. The soft material of the ring is drawn out between the lips during cell elongation. The septum (still composed of vesicles and microtubules) moves up the cell like a diaphragm, eventually reaching the bottom lip of the extended ring structure. Coalescence of vesicles then occurs in the septum to form the new transverse wall attached to the lower lip from the ring. The extensible material of the ring becomes the outer layer of the cell (mucilage?); the cell wall proper is evidently formed as a thinner layer on the inside edge of this diffuse material. It is therefore suggested...