IN prokaryotes, the genes for 5S, 16S and 23S ribosomal RNA are linked in a polycistronic transcriptional unit1,2. In eukaryotes, on the other hand, the analogous unit contains the genes for 5.8S, 18S and 28S rRNAs, whereas the 5S RNA genes are found elsewhere in the genome. The analogous positions occupied by prokaryotic 5S RNA genes and eukaryotic 5.8S RNA in their respective genomes prompted Doolittle and Pace to speculate that these two RNA classes, both of which are components of the large ribosomal subunit, might have a common evolutionary origin3. If these RNAs are in fact homologous, one would expect their sequences to bear some relationship to each other. We report here our evaluation of possible structural relationships between known sequences of 5.8S and 5S rRNA, and conclude that they have no common evolutionary origin.