Replication of ColE1-type plasmids is known to be regulated by a plasmid-specific RNA (RNA I), whose binding to the transcript (RNA II) from the primer promoter results in inhibition of formation of the primer for DNA replication. In this paper, it is shown that binding of RNA I to the homologous RNA II is inhibited by an RNA I specified by a plasmid of different compatibility. The inhibition is caused by the reversible interaction of RNA II with the heterologous RNA I, which competes with reversible interaction of the two homologous RNAs at a step preceding their stable binding. As predicted from these results, the copy numbers of both ColE1 and RSF1030 are increased when both plasmids are present in the same cell. The Rom protein enhances the reversible interaction.