Changes in the ultrastructure of the fission yeast Schizosaccharomyces pombe during the cell division cycle were analyzed by three-dimensional reconstruction of serial section electron micrographs of freeze-substituted cells. Cytoplasmic vesicles were found at the cell ends during interphase and at the equatorial zone of cells undergoing cytokinesis. Filasomes behaved in a similar but temporally retarded way to vesicles. Microfilament(mf)-associated granules were found attached to the plasma membrane at the growing ends. Microfilaments were identified against the plasma membrane and adjacent to developing septa. From these observations it is suggested that mf-associated structures such as filasomes constitute dense knots of actin network that function in localized cell wall growth by controlling the deposition of cytoplasmic vesicles. Dictyosomes occur as tubular and fenestrated cisternae with associated cytoplasmic vesicles. They were distributed uniformly in the cytoplasm and did not change significantly during the cell cycle. Changes in the three-dimensional localization of cytoplasmic microtubules and mitochondria are also described.