To persist in the hostile acidic environment of the stomach, Helicobacter pylori must survive acid shock and grow at acidic pH. Of a library of 1250 random mutants screened for isolates unable to grow at low pH, 10 mutants were detected that were unable to grow at pH 4.8. However, all 10 mutants were resistant to acid shock. Four mutants had an insertion in genes of unknown function. One mutant was affected in lepA, an orthologue of a membrane GTPase. Three mutants were disrupted in loci involved in the transport of H(+) ions or other cations (FRaseI, czcA, and aldo-keto reductase). Two mutants were affected in loci that contribute to acid resistance in other microorganisms (uvrA and atpF'). Thus, at least 10 loci not related to urease are essential for the growth of H. pylori under acidic conditions and should be critical for lifelong infection by this pathogen.