The Escherichia coli asr gene, like its homologues in other enterobacteria, is strongly induced by low external pH. The E. coli asr mutant shows weakened ability to adapt to acidic pH. This suggests that the asr gene product is important for enterobacterial species, both commensal and pathogenic, in overcoming acid stress in the stomach and subsequently colonizing the intestine. We examined the relative fitness of an E. coli asr mutant compared to a wild type, by feeding both strains simultaneously to mice and letting them colonize the intestine. Analysis of the bacteria after passage through the intestine showed up to five orders of magnitude less asr mutant than wild type. Transcomplementation of the asr gene on a plasmid partially restored the number of mutants. Similar competition in liquid media demonstrated that the asr mutant has reduced viability during long-term incubation in rich media, but is as fit as the wild type when bacteria are challenged in minimal medium. Competition carried out under different pH conditions proved that pH of the media was not the main determinant leading to the decreased fitness of the asr mutant. This suggests that the asr gene product is important for adaptation to stress conditions other than acidity, including long periods of starvation.