486 species of Pteridophyta were examined under trasmission electron microscope to investigate on the occurrence and morphological features of intranuclear proteinaceous inclusions. 315 species proved to contain nuclear inclusions, i.e. 64.81%. 301 out of the Filicophytina examined have proteinaceous nuclear bodies, giving 69.20%. Families which generally display archaic caracters usually lack nuclear proteinaceous inclusions altogether. Among these are: Huperziaceae, Lycopodiaceae and Isoëtaceae in the Lycophytina; Equisetaceae, in the Sphenophytina; Psilotaceae and Tmesipteridaceae in the Psilophytina; Botrychiaceae, Ophioglossaceae, Angiopteridaceae, Osmundaceae, Hymenophyllaceae, Azollaceae and Salviniaceae in the Filicophytina. The genusSelaginellaseems particularly interesting, both because it is the only Lycophytina with nuclear proteinaceous bodies and because the substructure of the inclusions is fibrillar, which so far has proved to be unique in the Pteridophyta. In the Polypodiaceae the absence of intranuclear paracrystals inPlatyceriumagrees with the thesis of CHING (1940) which places this genus in a distinct family, the Platyceriaceae.