I report here the ultrastructure of 29 ependymal tumors. The ultrastructural pattern was florid and characteristic with a picture dominated by the presence of microlumina, cilia with basal bodies (blepharoplast), microvilli and long, interdigitating intercellular junctions of the zonulae adherentes (adhesive plaque junctions) type. Tumor cells themselves were not particularly peculiar but they formed typical patterns of rosettes (so called mini- or ultrastructural rosettes) cell gatherings around small, electron-lucent lumina which are filled with numerous microvilli. Empty microlumina were rare. The apical and lateral portions of the cells surrounding microlumina were sealed by intercellular junctions which are long, tortuous and clearly different from the zonulae occludentes (tight junctions) of epithelial tumors. Clusters of apparently "redundant" junctions were occasionally visible comprising segments of different lengths. Ependymoma cells contained myriads of 10 nm intermediate filaments (glial filaments), occasionally forming thick bundles, virtually identical to those encountered in astrocytic tumors and forming an ultrastructural correlate for the GFAP immunostaining. The glycogen granules were often remarkably numerous. Numerous cilia, with a typical 9+1 pattern or with a distorted pattern were frequently observed in longitudinal or cross-sections.