ABSTRACT Alternative sigma factors of the extracytoplasmic function (ECF) subfamily are important regulators of stress responses in bacteria and have been implicated in the control of homeostasis of the extracytoplasmic compartment of the cell. This work describes the characterization of sigF , encoding 1 of the 13 members of this subfamily identified in Caulobacter crescentus . A sigF -null strain was obtained and shown to be severely impaired in resistance to oxidative stress, caused by hydrogen peroxide treatment, exclusively during the stationary phase. Although sigF mRNA levels decrease in stationary-phase cells, the amount of σ ^F protein is greatly increased at this stage, indicating a posttranscriptional control. Data obtained indicate that the FtsH protease is either directly or indirectly involved in the control of σ ^F levels, as cells lacking this enzyme present larger amounts of the sigma factor. Increased stability of σ ^F protein in stationary-phase cells of the parental strain and in exponential-phase cells of the ftsH -null strain is also demonstrated. Transcriptome analysis of the sigF -null strain led to the identification of eight genes regulated by σ ^F during the stationary phase, including sodA and msrA , which are known to be involved in oxidative stress response.