A Schizosaccharomyces pombe cps8 mutant, of which the gene encodes a mutant actin with an amino acid substitution of Asp for Gly(273) [J. Ishiguro and W. Kobayashi (1996) FEBS Lett. 392, 237-241], was used to determine the role of the actin cytoskeleton in cell wall formation. In the cps8 mutant cells, atomic force microscopic and scanning electron microscopic images showed abnormal depolarized and branched morphology. Fibrous material covered a part of the surface of growing cps8 cells. Transmission electron microscopic images showed variable thickness of the cell wall due to multilayering of cell wall materials, and aberrant multisepta due to diagonal growth of the primary septum, whereas the normal primary septum grows at a right angle from the cortex. This abnormal septum formation may induce abnormality of the cell with multinuclei and/or multisepta, caused by non-separation of daughter cells. These results indicate that actin plays an important role in cell wall and septum formation.