The numbers of potential response regulator genes were determined from the complete and annotated genome sequences of Archaea and Bacteria. The numbers of each class of response regulators are shown for each organism, determined principally from BLASTP searches, but with reference to the gene category lists where available. The survey shows that for Bacteria there is a link between the total number of potential response regulator genes and both the genome complexity (number of potential protein-coding genes) and the organism's lifestyle/habitat. Increasingly complex lifestyles and genome complexities are matched by an increase in the average number of potential response regulator genes per genome, indicating that a higher degree of complexity requires a higher level of control of gene expression and cellular activity. Detailed results of this study are available online at and.