The lux operon is an uncommon gene cluster. To find the pathway through which the operon has been transferred, we sequenced the operon and both flanking regions in four typical luminous species. In Vibrio cholerae NCIMB 41, a five-gene cluster, most genes of which were highly similar to orthologues present in Gram-positive bacteria, along with the lux operon, is inserted between VC1560 and VC1563, on chromosome 1. Because this entire five-gene cluster is present in Photorhabdus luminescens TT01, about 1.5 Mbp upstream of the operon, we deduced that the operon and the gene cluster were transferred from V. cholerae to an ancestor of Pr. luminescens. Because in both V. fischeri and Shewanella hanedai, luxR and luxI were found just upstream of the operon, we concluded that the operon was transferred from either species to the other. Because most of the genes flanking the operon were highly similar to orthologues present on chromosome 2 of vibrios, we speculated that the operon of most species is located on this chromosome. The undigested genomic DNAs of five luminous species were analysed by pulsed-field gel electrophoresis and Southern hybridization. In all the species except V. cholerae, the operons are located on chromosome 2.