RNA polyadenylation occurs not only in eukaryotes but also in bacteria. In prokaryotes, polyadenylated RNA molecules are usually degraded more efficiently than non-modified transcripts. Here we demonstrate that two transcripts, which were shown previously to be substrates for poly(A) polymerase I (PAP I), Escherichia coli lpp messenger RNA and bacteriophage lambda oop RNA, are polyadenylated more efficiently in slowly growing bacteria than in rapidly growing bacteria. Intracellular levels of PAP I varied in inverse proportion to bacterial growth rate. Moreover, transcription from a promoter for the pcnB gene (encoding PAP I) was shown to be more efficient under conditions of low bacterial growth rates. We conclude that efficiency of RNA polyadenylation in E. coli is higher in slowly growing bacteria because of more efficient expression of the pcnB gene. This may allow regulation of the stability of certain transcripts (those subjected to PAP I-dependent polyadenylation) in response to various growth conditions.