We synthesized a range of PQS (Pseudomonas quinolone signal; 2-heptyl-3-hydroxy-4(1H)-quinolone) analogues and tested them for their ability to stimulate MvfR-dependent pqsA transcription, MvfR-independent pyoverdine production, and membrane vesicle production. The structure-activity profile of the PQS analogues was different for each of these phenotypes. Certain inactive PQS analogues were also found to strongly synergize PQS-dependent pyoverdine production.