In order to monitor the fate of genetically manipulated fluorescent pseudomonads following release into the environment, a lacZY transposable cassette, lacking antibiotic resistance genes, was constructed using a pUT suicide plasmid delivery system. The resulting plasmid, pUTLacZY, can be easily used to generate lacZY marked pseudomonads without having to use antibiotic resistance determinants. The lacZY transposon generates random, stable transcriptional/translational fusions on integration into the target genome. Pseudomonas fluorescens strain F113 was marked with lacZY and was unaltered with respect to ecological fitness in the rhizosphere. Although lateral gene transfer of the chromosomally integrated lacZY marker could be detected in vitro, it was not detected in rhizosphere microcosms.