In general, plasmid vectors are not suitable for environmental studies. They are unstable in the absence of continued selection pressure and they can be horizontally transferred to other bacteria. In contrast, transposon vectors have enhanced stability and reduced horizontal transfer. Therefore Tn5 derivatives were constructed in which the antibiotic resistance genes are replaced by the pupA gene. The delivery vehicle pDK8 is based on a ColE1 replicon and cannot replicate in Pseudomonas. 20% of the transpositions of this transposon occurs without concomitant transposition of the transposase genes, resulting in very stable insertions. This Tn5 pupA transposon has been integrated into the chromosome of the fluorescent Pseudomonas strain Al24. The insertions mutants express the PupA protein and have the ability to metabolise iron(III) complexed to pseudobactin 358. This Tn5 pupA transposon vector therefore can be used to label strains with a selection marker or to introduce foreign genes into the chromosome of a strain. The strains constructed were tested in soil and rhizosphere in greenhouse trials. Data was obtained on colonization, survival, persistence and competition. No differences between wild type strains and genetically modified strains were observed sofar. The rhizosphere was found to be a major determinant in the selective amplification and survival of specific Pseudomonas spp.