Halogenated furanones produced by the benthic marine macroalga Delisea pulchra inhibit swarming motility of Serratia liquefaciens MG1. This study demonstrates that exogenously added furanones control transcription of the quorum sensing regulated gene swrA in competition with the cognate signal molecule N:-butanoyl-L-homoserine lactone. This in turn results in reduced production of the surface-active compound serrawettin W2, which is crucial for surface translocation of the differentiated swarm cells. It is demonstrated that furanones interfere with interspecies communication during swarming of mixed cultures and that the mode of interference in quorum-sensing control and interspecies communication is not through inhibition of autoinducer synthesis.