In recent years it has become clear that the production of N-acyl homoserine lactones (N-AHLs) is widespread in Gram-negative bacteria. These molecules act as diffusible chemical communication signals (bacterial pheromones) which regulate diverse physiological processes including bioluminescence, antibiotic production, plasmid conjugal transfer and synthesis of exoenzyme virulence factors in plant and animal pathogens. The paradigm for N-AHL production is in the bioluminescence (lux) phenotype of Photobacterium fischeri (formerly classified as Vibrio fischeri) where the signalling molecule N-(3-oxohexanoyl)-L-homoserine lactone (OHHL) is synthesized by the action of the LuxI protein. OHHL is thought to bind to the LuxR protein, allowing it to act as a positive transcriptional activator in an autoinduction process that physiologically couples cell density (and growth phase) to the expression of the bioluminescence genes. Based on the growing information on LuxI and LuxR homologues in other N-AHL-producing bacterial species such as Erwinia carotovora, Pseudomonas aeruginosa, Yersinia enterocolitica, Agrobacterium tumefaciens and Rhizobium leguminosarum, it seems that analogues of the P. fischeri lux autoinducer sensing system are widely distributed in bacteria. The general physiological function of these simple chemical signalling systems appears to be the modulation of discrete and diverse metabolic processes in concert with cell density. In an evolutionary sense, the elaboration and action of these bacterial pheromones can be viewed as an example of multicellularity in prokaryotic populations.