Two genetically independent Chlamydomonas mutants, oca1 and oca2, that display abnormal cell division were isolated by DNA insertional mutagenesis. The culture of these mutants contained large abnormally-shaped cells with multiple pairs of flagella. DAPI staining showed that those aberrant cells carried the same number of nuclei as that of flagella pairs. Time-lapse video microscopy revealed the following characteristics of the cell division process in the mutants: i) although the mutants, like wild-type cells, had a potential to divide into eight daughter cells by successive three rounds of division cycle, they frequently failed to cleave and formed a fused cell in the course of division; ii) a cell often grew into an extremely large cell with many pairs of flagella; iii) the large cell was suddenly divided into a number of daughter cells by simultaneously forming multiple cleavage furrows; iv) alternatively, an extremely large cell stopped dividing although many cleavage furrows were formed on its surface. These observations suggest that these mutants are partially deficient in the progression of furrowing, and that Chlamydomonas is capable of undergoing cytokinesis between many pairs of nuclei simultaneously, as in the cellularization process of insect eggs.