Production of light by the marine bacterium Vibrio fischeri and by recombinant hosts containing cloned lux genes is controlled by the density of the culture. Density-dependent regulation of lux gene expression has been shown to require a locus consisting of the luxR and luxI genes and two closely linked divergent promoters. As part of a genetic analysis to understand the regulation of bioluminescence, we have sequenced the region of DNA containing this control circuit. Open reading frames corresponding to luxR and luxI were identified; transcription start sites were defined by S1 nuclease mapping and sequences resembling promoter elements were located.