Treatment of cells and purified cell walls of the fission yeast Schizosaccharomyces pombe with primuline reveals the septum as a bright fluorescent band. When polysaccharides containing (1----3)-beta-, (1----6)-beta- or (1----3)-alpha-glucosidic linkages are treated with primuline, only those molecules containing chains of (1----3)-beta-glucosyl residues are stained. This implies that (1----3)-beta-glucan is present in the septum of Schiz. pombe as the main constituent.