Compared to the known method of conjugation the frequency of transposon mutagenesis following conjugation was enhanced 11-fold by two hours of pre-incubation of recipient Pseudomonas viridiflava 1 on conjugation media. The increased frequency was ƒ = 1.3 × 10 −4 . In other species of Pseudomonas, Pectobacteira and Xanthomonas high rates of transposon mutants were similarly obtained; however, in these strains the increased frequency was less than 5-fold.