Skeletogenesis in polycystine radiolarians was detected, using a fluorescent compound called PDMPO (2-(4-pyridyl)-5-[(4-(2-dimethylaminoethylaminocarbamoyl)methoxy)-phenyl] oxazole). This compound binds under acidic conditions with silica in the silica deposition vesicle (SDV) of radiolarians, and only newly added silica emits a green fluorescence under ultraviolet light. We used cells fixed with formaldehyde of two species of spherical Spumellaria (Hexacontium philosophica and Rhizosphaera trigonacantha) and three species of cyrtid Nassellaria (Anthocyrtidium angulare, Clathrocyclas (?) cassiopeiae, and Lophophaena hispida), in which green fluorescence occurred in the entire skeleton outside the endoplasm. In contrast, the conical nassellarian species Eucyrtidium hexagonatum emitted green fluorescence only on the cephalo-thorax-abdominal part of the skeleton, and on part of the inner ring at the segmental suture, and the margins of pores, suggesting partial biological silicification. Two flat spumellarian species (Dictyocoryne profunda and Rhopalastrum elegans) showed bright green fluorescence over the full skeleton in some cells, but other cells did not fluoresce at all, even though young cells in good condition were tested, suggesting that intermittent growth occurs in these species, as known from a controlled incubation experiment. We did not observe development of new skeletons, but "skeletal thickening growth" (STG), defined as silica deposition over the surface of an existing skeleton, occurred to varying degrees in the polycystines, and may result in the formation of lamellar structures, as observed in sectioned skeletons of polycystines. (C) 2010 Elsevier B.V. All rights reserved.