Photodynamic treatment of Kluyveromyces marxianus with the sensitizer Toluidine blue leads to the loss of colony forming capacity. In this paper, the influence of this treatment on the barrier properties of the plasma membrane has been studied. Photodynamic treatment with the dye Toluidine blue resulted in efflux of potassium ions and E260-absorbing material. Moreover, cells became stainable with erythrosine. It is concluded that the permeability change induced by photodynamic treatment proceeds in an all-or-none fashion. Treatment of this yeast strain, with the dye and light, also induced a diminution of the cell volume. This process is most likely not coupled to the cellular potassium content, but rather to the integrity of the vacuole. These data suggest that the vacuole has an important function in the maintenance of cell volume. Finally, it was observed that the loss of cell viability was not induced by the all-or-none loss of barrier properties.