A novel method is described which uses aniline blue for the specific fluorescent staining of the septa of dividing cells of the fission yeast, Schizosaccharomyces pombe. It gives the same results with live and fixed cells. In fixed or, more generally, dead cells there is no staining of the cytoplasm: this renders aniline blue superior to other dyes previously used to stain the septum of S. pombe. This feature allows quantitative analysis of the septum index for fixed samples and, therefore, makes aniline blue the stain of choice for cell cycle kinetic studies.