The usefulness of oxonol (bis-(1,3-dibutylbarbituric acid)trimethine oxonol) as a generally applicable indicator of bacterial viability was investigated using untreated and killed cultures of a variety of bacterial genera. Killing methods involved either heat or bactericidal antibiotics. For all strains tested, the fluorescent dye showed significantly more intense staining of killed than untreated cells. The sensitivity of Aeromonas salmonicida to gentamicin was assessed using oxonol. Although the bacterium was shown to be sensitive to the antibiotic, there was a delay between the time cells lost culturability, as judged by numbers of colony forming units, and that for which a dead cell population could be detected by flow cytometry.