The molecule 2-heptyl-3-hydroxy-4-quinolone (referred to as the Pseudomonas quinolone signal, or PQS) is produced by Pseudomonas aeruginosa as part of its quorum sensing circuit, and has been shown to influence a variety of processes in this bacterium, including the production of siderophores and secondary metabolites, virulence determinants and biofilm development. In this report we present evidence of the effect of PQS as an interspecies signal with a negative impact on the multicellular behaviour of Pseudomonas putida KT2440. PQS reduces biofilm formation and swarming motility, and interferes with iron uptake by this bacterium. Addition of PQS also causes changes in the transcription of several P. putida genes, indicating a specific response to the signal molecule, which is not produced by strain KT2440. Among the genes with increased expression in response to PQS is PP1563, which forms part of a large prophage cluster (PP1532-PP1584); consistently, phage-mediated lysis of some cells in the population was observed in the presence of PQS. Overall, these data indicate that PQS may be used by P. aeruginosa as a chemical weapon against potential competitors.