A new procedure for isolating pellicles from Tetrahymena yields stable whole pellicles in good yield. Electron microscopy of fixed pellicles showed inner membranes and basal bodies but no mitochondria. Sucrose gradient sedimentation of pellicle extracts labelled with [ 3 H]uridine showed an increase in the ratio 17 s RNA to 25 s RNA and little 4 s RNA. The method of long term labelling presumably excluded messenger RNA. The technique of RNA/DNA hybridization in the presence of competing RNA showed that 35% of the pellicle RNA which could hybridize (C 0 t 2-3) to DNA did not contain sequences in common with ribosomal RNA. It is proposed that a stable RNA which is not mitochondrial RNA, transfer RNA or ribosomal RNA is associated with the pellicle. Fixed pellicles stained with acridine orange showed brilliant yellow-green fluorescence at the basal bodies. RNase reduced or abolished fluorescence; DNase, histone, and lysozyme had no effect. Mercaptoethanol changed the colour of the fluorescence of basal bodies from yellow-green to orange, while not changing yellow nuclear fluorescence. Fluorescence of basal bodies was seen in pellicles isolated from log-phase cells and also in synchronized cells. It is proposed that basal bodies contain, or are associated with, single-stranded RNA held in a rigid configuration by protein. The implications of this proposal are discussed.