Plasmid pC1 of Delftia acidovorans CA28, encoding the oxidative deamination of 3-chloroaniline (3-CA), was tagged with a mini-Tn5 transposon containing the gfp gene. The labelled plasmid, designated pC1gfp, was subsequently transferred to Pseudomonas putida UWC3 and the plasmid transfer from this donor to the bacterial community in activated sludge was studied. Mating experiments were performed on both Luria-Bertani agar plates and directly in liquid activated sludge. Green fluorescent colonies appearing on mineral medium containing 3-CA as sole nitrogen source were picked up and verified to be true pC1gfp-harbouring transconjugants. REP- and BOX-PCR genomic fingerprinting revealed a large diversity in the transconjugant collection, indicating the occurrence of multiple plasmid transfer events. Remarkably, Luria-Bertani agar plate matings yielded a different set of transconjugants compared to plasmid transfers directly in liquid activated sludge. From the plate matings, mainly Aeromonas sp. was isolated, whereas D. acidovorans strains were predominant in the transconjugant collection from liquid activated sludge. The majority of the isolates showed 3-CA deamination, probably to chlorocatechol, but several pC1gfp-harbouring transconjugants performed a rapid and complete 3-CA degradation.