OUR understanding of the evolution of the cell will depend largely on our gaining an understanding of the evolution of the cell's translation apparatus, and in this context I wish to discuss the recent finding that the RNA sequences (seen in a Tl ribonuclease digest) surrounding the methylated bases in the Escherichia coli 23S ribosomal RNA (rRNA) are all repeated twice in the molecule1. This has been interpreted to mean either that the 23S rRNA molecule is actually a dimer of two identical halves, or that the 23S cistron has evolved through a gene duplication and joining mechanism1. The dimer interpretation seems to be ruled out by the fact that there are many more kinds of sequences in the 23S rRNA than there are in the 16S rRNA, and so, reasonably, the former should be larger than the latter, if most sequences were singly represented in each2. In. view of the evolutionary significance of the second possibility, however, I wish to point out that there is at least one other reasonable interpretation of these data (which does not invoke gene duplication) and so they cannot be taken as evidence for an origin of the 23S rRNA cistron by a duplication and joining involving a smaller cistron.