Cell division of the fission yeast, Schizosaccharomyces pombe, is reversibly inhibited by the antimitotic agent Colcemid (N-deacetyl-N-methylcolchicine) in nutrient medium. Cell growth continiues until all cells become nonseparating cell doublets in a V configuration. Mutants have been isolated capable of uninhibited growth in the presence of concentrations of Colcemid mycostatic for the parent strain.