The stress-responsive, alternative sigma factor σB has been described in members of three Gram-positive genera, Bacillus, Listeria, and Staphylococcus. In these bacteria, σB appears to play an important role in facilitating rapid adaptation to and survival in stressful environments. σB activity is regulated through a complex system of phosphatases and kinases encoded by rsb (regulator of sigma B) genes. We describe the sigB operon structure for the facultative intracellular pathogen Listeria monocytogenes and apply this sequence as well as other previously described sigB operon sequences to probe the evolution and functional conservation of the σB stress response system among different Gram-positive bacteria. While σB as well as two Rsbs (RsbS and RsbT) are highly conserved (73%, 84%, and 79% average amino acid [aa] identities, respectively), the predicted aa sequences of the other Rsb proteins showed less conservation (62-71% aa identities). Furthermore, the sigB operon structure varies among bacterial species. Bacterial species differ in the numbers and identities of rsb genes encoded in their genomes. We thus conclude that the σB stress-response system as represented by the sigB operon has diverged in both its overall components as well as in the sequences of its individual proteins, even among closely related bacterial species. Differential evolution of this stress response system among various genera may represent a strategy that enables bacteria to adapt cellular response and survival systems to a variety of stress conditions.