An ampicillin transposon Tn901 was used as a "mutagen" to isolate insertion mutants of the bacteriocinogenic plasmid Clo DF13. By combining the obtained heteroduplex and restriction maps of the Clo DF13::Tn901 plasmids (van Emboden et al., 1977b) with their polypeptide pattern in minicells, we were able to map five genes on the Clo DF13 genome. These five genes designated A (cloacin gene), B, C, D, and G cover 55% of the coding capacity of Clo DF13 DNA. Since integration of Tn901 within these five genes did not result in a loss of the Clo DF13::Tn901 plasmids involved, it is suggested that these genes do not play an essential role in the maintenance of these plasmid insertion mutants. In addition, the described methods allowed us to indicate the initiation site of cloacin synthesis and to propose the counter-clockwise direction of transcription of the cloacin gene. The Tn901 DNA directed the synthesis of at least three polypeptides one of which is shown to be a TEM-1 beta-lactamase.