Density‐dependent expression of luminescence in Vibrio harveyl is regulated by the concentration of extracellular signal molecules (autoinducers) in the culture medium. One signal‐response system is encoded by the luxL,M,N locus. The luxL and luxM genes are required for the production of an autoinducer (probably β‐hydroxybutryl homoserine lactone), and the luxN gene is required for the response to that autoinducer. Analysis of the phenotypes of LuxL,M and N mutants indicated that an additional signal‐response system also controls density sensing. We report here the identification, cloning and analysis of luxP and luxQ, which encode functions required for a second density‐sensing system. Mutants with defects in luxP and luxQ are defective in response to a second autoinducer substance. LuxQ, like LuxN, is similar to members of the family of two‐component, signal transduction proteins and contains both a histidine protein kinase and a response regulator domain. Analysis of signalling mutant phenotypes indicates that there are at least two separate signal‐response pathways which converge to regulate expression of luminescence in V. harveyl.