Bacteriophage Mud (Casadaban and Cohen 1979) was used to bring the transcription of the gene for beta galactosidase (lacZ) under the control of the promoter of the structural gene for colicin Ib (cia(Ib)) on a derivative of the Col plasmid Col-Ib.P9. Transcription of this fusion operon was stimulated by agents which damaged cellular DNA (mitomycin C, bleomycin and colicin E2). Increased transcription of the cia-lacZ operon could be detected within 13 min of the addition of these agents. In a strain bearing the tif-1 (recA441) mutation, constitutive expression of the SOS DNA repair system at 42 degree C also increased transcription of the cia-lacZ operon. Transcription of the cia-lacZ operon was also stimulated by inhibition of DNA gyrase activity with nalidixic acid but not with novobiocin. Transitory inhibition of protein synthesis with chloramphenicol or by proline starvation of a proline auxotroph did not stimulate cia-lacZ transcription. Transcription of the cia-lacZ operon was substantially reduced in the presence of a recA mutation, but was largely unaffected by a mutation in recB affecting the RecBC DNase or by catabolite repression. Control experiments in which the production of colicin Ib was measured confirmed that the experiments with the fusion operon gave an accurate indication as to the activity of the wild type cia gene except for the effect of catabolite repression, where we observed up to 99% reduction in colicin Ib production in strains carrying mutant crp or cya alleles. The overall results confirm previous suggestions that there was considerable similarity between the regulatory systems controlling production of colicins and the repressor-dependent regulation of lambdoid prophage induction.