It has long been appreciated that certain groups of bacteria exhibit cooperative behavioral patterns. For example, feeding and sporulation of both myxobacteria and actinomycetes seem optimized for large populations of cells behaving almost as a single multicellular organism. The swarming motility of micro-organisms such as Vibrio parahaemolyticus and Proteus mirabilis provides another excellent example of multicellular behavior among bacteria (2). Intercellular communication likewise has been appreciated for several years in Vibrio fischeri, Myxococcus xanthus, Bacillus subtilis, Streptomyces spp., the eukaryotic slime mold Dictyostelium discoideum, and other species (44). Here we first review how the marine luminescent bacterium V. fischeri uses the LuxR and LuxI proteins for intercellular communication and then describe a newly discovered family of LuxR and LuxI homologs in diverse bacterial species.