The ColE1 Rom protein contributes to copy number control by affecting the rate of formation of a complex between RNA II, the precursor of the primer for DNA replication, and RNA I, a small RNA complementary to the 5′ end of RNA II. Interaction of RNA I with RNA II can affect plasmid copy number by preventing primer formation. Although the RNA I and RNA II transcripts have been well characterized, the rom mRNA has not previously been detected. We have now identified the rom mRNA, and determined its start site both in vitro and in vivo. We also have found that the rom mRNA terminates at any of several sites, and that its synthesis is not autoregulated by the Rom protein. By using an internal standard RNA to estimate the efficiency of detection of the rom mRNA, its level was determined to be about 1 molecule per cell. © 1989 IRL Press at Oxford University.