A method of transductional complementation was developed in Pseudomonas aeruginosa to identify the cistrons involved in the conjugal transfer of the wide host range R plasmid R18. This used the P. aeruginosa bacteriophage E79tv-2 and has led to the identification of eight tra cistrons encoded by this plasmid. Plasmids mutant in six cistrons, traA, traB, traC, traD, traE, and traG were resistant to donor-specific phage (Dps-) while traF and traH mutant plasmids retained phage sensitivity. Some traB mutants were unable to inhibit the replication of phage G101 (Phi(G101)-) while some were also deficient in entry exclusion (Eex-). Two traB mutants which were also Eex- were suppressible by an amber suppressor. Three tra mutants selected directly as being Phi(G101)- were found to be also Dps-Eex- and mutant in traB. These data suggest a relationship between traB, Eex, and Phi(G101). In order to facilitate future genetic comparison of the tra genes of R18 and other wide host range plasmids and the role of the host in conjugation, R18 DNA was compared with that of RP4, by restriction enzyme fragment patterns and found to be identical.