The first phenotype described for mutations in the Escherichia coli rpoS gene was hypersensitivity to near-ultraviolet radiation and to its oxidative photoproduct, hydrogen peroxide. Initially named nur, this gene is now known to code for a sigma factor, and has acquired new names such as katF and rpoS. The role of its protein product (sigma-38) is to regulate a battery of genes as cells enter and rest in stationary phase. Some of the gene products are involved in protection against oxidants (e.g., catalases) and repair of oxidative damage (e.g., exonuclease III). Sigma-38 may also modulate transcription of certain growth phase genes, including hydroperoxidase I and glutathione reductase. Sigma-38 activity is regulated at transcriptional, translational, and protein stabilization levels. This review describes the complex mechanisms whereby sigma-38 controls various genes, the interaction of sigma-38 with other regulators, and a possible role of sigma-38 in bacterial virulence.