THERE is no doubt that the large ribosomal RNAs play specific roles in ribosome function1. Yet the thrust of experimentation during the past 5 years indicates clearly that the biologist tends to view these roles as structural, function being reserved by and large for the ribosomal protein components2,3. Whether or not this prejudice can be maintained remains to be seen. In any case, the existence of an approximate sequence for a 16S rRNA provides a basis for detailed characterisations of this molecule3,4. Here we begin to define 16S rRNA function by localising the major conserved regions in the molecule through a comparative analysis of 27 prokaryotic 16S rRNA primary structures.