OBJECTIVES: To search for plasmid-mediated qnr genes among waterborne environmental Aeromonas spp. recovered from Switzerland. METHODS: Isolates presenting MICs of nalidixic acid or ciprofloxacin > or = 1 mg/L were screened for qnr genes by a multiplex PCR approach followed by sequencing. Plasmids were transferred by transformation, and further analysis of the genetic structures surrounding the qnrS2 gene was carried out by PCR and sequencing. RESULTS: A qnrS2 gene was identified from a single Aeromonas allosaccharophila isolate (Lugano lake, Lugano), as part of a mobile insertion cassette located on a broad host range IncU-type plasmid. This plasmid co-harboured a class 1 integron containing the aac(6')-Ib-cr, bla(OXA-1), catB3 and arr-3 gene cassettes. CONCLUSIONS: These findings strengthen further the role of Aeromonas spp. as a reservoir of antimicrobial resistance determinants in the environment.