Density-dependent expression of luminescence in Vibrio harveyi is regulated by the concentration of extracellular signal molecules (autoinducers) in the culture medium. A recombinant clone that restored function to one class of spontaneous dim mutants was found to encode a function required for the density-dependent response. Transposon Tn5 insertions in the recombinant clone were isolated, and the mutations were transferred to the genome of V. harveyi for examination of mutant phenotypes. Expression of luminescence in V. harveyi strains with transposon insertions in one locus, luxO, was independent of the density of the culture and was similar in intensity to the maximal level observed in wild-type bacteria. Sequence analysis of luxO revealed one open reading frame that encoded a protein, LuxO, similar in amino acid sequence to the response regulator domain of the family of two-component, signal transduction proteins. The constitutive phenotype of LuxO- mutants indicates that LuxO acts negatively to control expression of luminescence, and relief of repression by LuxO in the wild type could result from interactions with other components in the Lux signalling system.