2-(4-pyridyl)-5-[(4-(2-dimethylaminoethylaminocarbamoyl)methoxy)-phenyl] oxazole (PDMPO) is a fluorescent compound that accumulates in acidic cell compartments. PDMPO is accumulated with silica under acidic conditions, and the newly developed silica skeletons show green fluorescent light. This study is the first to use PDMPO in polycystine radiolarians, which are unicellular planktonic protists. We tested Acanthodesmia sp., Rhizosphaera trigonacantha, and Spirocyrtis scalaris for emission of green fluorescence. Entire skeletons of Acanthodesmia sp. and Sr. scalaris emitted green fluorescent light, whereas only the outermost shell and radial spines of Rz. trigonacantha showed fluorescence. Two additional species, Spongaster tetras tetras and Rhopalastrum elegans did not show fluorescence. Green fluorescence of the entire skeleton is more like the "skeletal thickening growth" defined by silica deposition throughout the surface of the existing skeleton. The brightness of the fluorescence varied with each cell. This difference in fluorescence may reflect the rate of growth in these cells. Green fluorescence in PDMPO-treated polycystines suggests the presence of similar metabolic systems with controlled pH. The results of this study shed light not only on the feasibility of using PDMPO in diatoms and siliceous sponges, but also in polycystines. Further application of PDMPO could refine polycystine skeletogenic hypotheses and offer new insight on the physiological functions of silica assimilation. © The Plankton Society of Japan.