Vibrio vulnificus is an opportunistic pathogen that exhibits numerous virulence factors, including the secretion of a zinc metalloprotease and the production of a capsule. We have cloned and sequenced a gene from V. vulnificus that is a homologue of the positive transcriptional regulator, luxR, of the lux operon in Vibrio harveyi. This gene encodes a putative, single complete open reading frame designated smcR, which shares greater than 75% nucleotide identity with luxR of V. harveyi. The deduced amino acid sequence of the putative SmcR protein is more than 90% identical and 95% similar to that of LuxR of V. harveyi, suggesting that V. vulnificus possesses a member of the family of signal-response genes recently described in Vibrio cholerae and in Vibrio parahaemolyticus. Our data also demonstrate that, in addition to V. vulnificus, all six Vibrio spp. tested contained genes that hybridized with the luxR probe. We also present evidence that this regulatory protein was inherited from a common ancestor, and that the gene is ancient and widespread in marine Vibrio spp.