SUMMARY A mutant of Chlamydomonas reinhardii with abnormal cell division is de- scribed. Cultures of the mutant consist of cells with between one and about seven nuclei. The number of pairs of flagella per cell is the same as the number of nuclei, except in large moribund cells which lack flagella. Pyrenoid divi- sion lags behind nuclear division in some cells. The expression of the muta- tion is modified by the age of the culture, pH of the environment or by the presence of certain other mutations. Genetic analysis is slightly complicated by the presence of multinucleate gametes, but it is shown that cytokinesis deficiency is inherited as a single gene mutation which maps close to the ac 29 locus on linkage group VI.