Fluorescein diacetate (FDA) was applied to the viability assessment of spores of Aspergillus niger, Rhizopus stolonifer, Fusarium oxysporum, and Penicillium citrinum. The fluorescence of individual cells was quantitated with a charge coupled device (CCD) detector. When staining was carried out in a phosphate buffer solution (10 mM, pH 7.0), weak or no fluorescence was emitted from viable spores of A. niger and R. stolonifer, which made it difficult to distinguish between viable (nontreated) and nonviable (heat treated at 90 degrees C for 30 min) spores. The addition of NaCl, KCl, or MgCl2 to the staining solution caused an increase in the fluorescence intensity of A. niger viable spores, from which nonviable spores could be distinguished. The same effect of NaCl was observed in staining the spores of other species.