We have demonstrated that the replication of the oriC plasmid, carrying the replication origin of the Escherichia coli chromosome, is inhibited by transcriptional readthrough from an oriC flanking region of the plasmid. This was drawn from an examination of the replication of an oriC plasmid, pKZ4, which bears the lacOP segment at the right-hand side of oriC (the asnA side) in such an orientation that transcription from the lac promoter proceeds towards oriC. Replication of pKZ4 was found to be drastically inhibited by inducing transcription from the lac promoter with IPTG, an inducer of the lactose operon. When trp transcription attenuator termination sequences were inserted near the right-hand end of the oriC region of pKZ4, the replication of the plasmid became considerably insensitive to the inhibitory effect of IPTG. This indicates that the inhibition is due to the frequent leftward transcription, which initiates at the lac promoter and proceeds into the oriC region. Since IPTG inhibits the replication of pKZ4, but not that of another coexisting oriC plasmid which is devoid of the lacOP segment, the replication inhibition is judged to act only in cis. Transcription from the promoter of the chloramphenicol resistance gene also caused the inhibition of oriC plasmid replication.