Pseudomonas aeruginosa is able to persist during feast and famine in many different environments including soil, water, plants, animals and humans. The alternative sigma factor encoded by the rpoS gene is known to be important for survival under stressful conditions in several other bacterial species. To determine if the P. aeruginosa RpoS protein plays a similar role in stationary-phase-mediated resistance, an rpoS mutant was constructed and survival during exposure to hydrogen peroxide, high temperature, hyperosmolarity, low pH and ethanol was investigated. Disruption of the rpoS gene resulted in a two- to threefold increase in the rate of kill of stationary-phase cells. The rpoS mutant also survived less well than the parental strain during the initial phase of carbon or phosphate-carbon starvation. However, after 25 d starvation the remaining population of culturable cells was not significantly different. Stationary-phase cells of the RpoS-negative strain were much more stress resistant than exponentially growing RpoS-positive cells, suggesting that factors other than the RpoS protein must be associated with stationary-phase stress tolerance in P. aeruginosa. Comparison of two-dimensional PAGE of the rpoS mutant and the parental strain showed four major modifications of protein patterns associated with the rpoS mutation.