Polyadenylation controls mRNA stability in procaryotes, eucaryotes, and organelles. In bacteria, oligo(A) tails synthesized by poly(A) polymerase I are the targets of the 3'-to-5' exoribonucleases: polynucleotide phosphorylase and RNase II. Here we show that RNase II very efficiently removes the oligo(A) tails that can be used as binding sites by PNPase to start degradation of the rpsO mRNA. Both enzymes are impeded by the secondary structure of the transcription terminator at the 3' end of the mRNA. RNase II mostly generates tailless transcripts harboring 2 unpaired nt downstream of the transcription terminator hairpin, whereas PNPase releases molecules that exhibit a single-stranded stretch of 5-7 nt terminated by a tail of 3-5 As. The rpsO mRNAs whose oligo(A) tails have been removed by RNase II are more stable than oligoadenylated molecules that occur in strains deficient for RNase II. Moreover, the rpsO mRNA is stabilized when RNase II is overproduced. This modulation of mRNA stability by RNase II is only observed when poly(A) polymerase I is active. These in vivo data demonstrate that RNase II protects mRNAs ending by stable terminal hairpins, such as primary transcripts, from degradation by poly(A)-dependent ribonucleases.