Flow cytometry is a rapid method for measuring the optical properties of individual cells. The technique has found great utility in the study of mammalian cells, but microbiological applications have been more limited. We here show that UV-excited fluorescent whitening agents, in particular Tinopal CBS-X, are effective stains for both vegetative microbial cells and for spores of Gram-positive bacteria. Pretreatment of samples with ethanol speeds the staining process. Under favourable conditions, Tinopal CBS-X may be used to discriminate among organisms, a fact that may be useful when screening for a target microorganism against a high biological background.