A fluorescence-staining method using the magnesium salt of 1-anilino-8-naphthalene sulfonic acid is described. It allows the numbers and types of viable microorganisms in soil sections to be determined by direct microscopy. About 8% of the bacteria in the soils studied were able to divide when a nutrient solution was applied. A higher percentage of the fungal propagules in these soils were viable. The effects of environmental factors, chemical compounds, and soil amendments on the ability of soil microorganisms to undergo division or mycelial extensionin situcan also be determined.