We have studied the sequence of 5S ribosomal RNA of Proteus vulgaris. Although several doubts remain, it seems that no more than 8 per cent of the nucleotides differ from those of Escherichia coli 5S RNA. Of the ten bases differing from Escherichia coli 5S RNA sequence, three changes take place in one of the six positions which are known as intercistronic mutation points in Escherichia coli 5S RNA. We can conclude that they are "hot spots" of mutation.