In addition to tRNA and 5S RNA, Escherichia coli contains several other small, stable RNA species; these are M1, 10Sa, 6S, and 4.5S RNA. Although these RNAs are initially synthesized as precursor molecules, relatively little is known about their maturation. The data presented here show that 3' exoribonucleolytic trimming is required for the final maturation of each of these molecules. As found previously with tRNA, but not 5S RNA, any one of a number of exoribonucleases can carry out the trimming reaction in vivo, although RNases T and PH are most effective. In their absence, large amounts of immature molecules accumulate for most of the RNAs, and these can be converted to the mature forms in vitro by the purified RNases. A model is proposed that identifies a structural feature present in all the small, stable RNAs of E. coli, and describes how this structure together with the RNases influences the common mechanism for 3' maturation.