The extrachromosomal genes coding for ribosomal RNA (rDNA) in the ciliated protozoan Tetrahymena thermophila were studied with respect to sequences occurring at their termini. The linear rDNA molecules had previously been shown to be palindromic (Karrer and Gall, 1976; Engberg et al., 1976). Within the terminal rDNA fragment produced by restriction endonuclease digestion, a tandomly repeated hexanucleotide sequence 5′ (C-C-C-C-A-A)_n 3′ was found, where n is between 20 and 70. This fragment was heterogeneous in length as judged by gel electrophoresis. The repeating sequence was preferentially synthesized when rDNA was used as the template by Escherichia coli DNA polymerase I. Initiation occurred at specific single-strand discontinuities, probably one-nucleotide gaps, found every few repeats on the C-C-C-C-A-A strand. At least one discontinuity is present on the G-G-G-G-T-T strand. Experiments with T4 DNA polymerase suggested that there are no free cohesive ends on the rDNA of the kind found in bacteriophage λ DNA. The orientation of the strands carrying the repeating hexanucleotide sequence was determined, and a model for the termini of the rDNA based on these findings is presented.