Plasmid ColIb-P9 is a 93.2 kb self-transmissible plasmid, belonging to the I1 incompatibility group. Whilst much data had been gained concerning the molecular biology of conjugation mediated by this plasmid, a lack of information exsisted concerning the genetic organisation of the transfer genes. A physical map of the plasmid was constructed by detailed restriction analysis of DNA fragments sub-cloned from ColIb-P9. These fragments were also used to locate the positions of the transfer gene sog and the origin of transfer. Transposons Tn5 and Tnl723 were used to construct insertion mutants at defined points in ColIb-P9 and the effect of these on the expression of various transfer-related functions was studied. Using this technique, the probable location of the genes encoding the thick and thin sex pili were identified and also the site of the plasmid-encoded nuclease gene. The exact location of the entry exclusion gene was also determined. Complementation studies using the sub-cloned fragments of ColIb-P9 and a set of cosmid-clones generated from ColIbdrd-1 indicated that a positive regulator of the expression of the transfer genes exsisted and that this was composed of two genetically distinct elements. Studies involving wild type ColIb-P9 (drd+) indicated that this positive regulatory system is subject to negative control in cells containing the drd+ plasmid. The information gained from these studies was combined into a model of the organisation of the transfer genes of ColIb-P9. This defines at least three separate Tra regions, covering some 50 kb of the plasmid, with the origin of transfer located at one end of the transfer region.