We have been able to demonstrate RNA which is present only in colicinogenic cells by using purified Col Ib-P9 plasmid DNA and [32P]-pulse labeled RNA in DNA-RNA hybridization experiments. The relative amount of plasmid specific RNA in colicinogenic cells can be increased by treatment with mitomycin C and correlates with the increase in colicin producing cells. Using competition hybridization we have shown that the Col Ib-P9 plasmid specific RNA from uninduced cells contains the same species of RNA found in induced cells and vice versa. There is a requirement of 3′, 5′-cyclic AMP and 3′, 5′-cyclic AMP receptor protein for both colicin Ib-P9 synthesis and plasmid specific RNA synthesis. It is suggested that colicin Ib synthesis is under transcriptional control.