The ability to withstand an acid-challenge of pH 2.5 or less by Shigella flexneri is a necessary trait for virulence and is generally believed to be restricted to the stationary-phase of growth. Earlier reports indicated the glutamate-dependent acid-resistance (GDAR) system of S. flexneri is under the regulation of rpoS, the gene encoding alternative sigma factor that is induced in the stationary-growth phase. The present study reports that unlike Escherichia coli O157:H7, S. flexneri cells when grown in minimal medium, require acid-induction in the stationary-growth phase for a functional GDAR. When grown on complex medium at pH 5.5, GDAR of S. flexneri was vigorous compared to the cells grown at pH 7.5. No acid-induction was required for the stationary phase E. coli cells grown on either minimal or complex growth media. Distinct differences in the gadA, gadBC, gadE, and hdeA (but not in rpoS) transcript levels were observed in the stationary-growth phase cells between the two pathogens grown on minimal medium. Additionally, rpoS-independent acid-induction of GDAR in the logarithmic growth phase that has been recently observed in E. coli strains [FEMS Microbiol. Lett. 227 (2003) 39–45] was not detected in the S. flexneri rpoS mutant. Although some differences in the DNA sequence at the upstream regulatory elements of gadBC were noticed, they do not appear to be significant and involvement of additional regulators in S. flexneri is anticipated, which also may explain the observed differences in the GDAR of two enteric pathogens.