Fluorescence was observed under light microscope in living cells and cell mass of Dictyostelium mucoroides. The fluorescence was localized in the vacuoles of live vegetative cells. While the cell mass of D. discoideum does not have a stalk during migration period, the cell mass of D. mucoroides has a stalk that forms at the beginning of the migration period. We were able to observe a preferential loss of the fluorescent vacuoles from the cells of the stalk and from the stalk-forming cells at the tip region of the slug. Although the fluorescence was also present in the mature spore mass of D. mucoroides, the fluorescence was not observed in the spores, but rather in the spaces between the spores within the spore mass. The fluorescent vacuoles in the cells of vegetative stage and of migrating slug stage may be related to the interspore fluorescence in the spore mass. Possible roles of the fluorescent substance(s) in amoebae, slugs and spore masses were discussed.