Shigella boydii CDPH (Chicago Department of Public Health) serotype 18 was implicated in an outbreak of foodborne illness in 1998. The suspected food vehicles were parsley and cilantro imported from Mexico used to prepare bean salad. Previous studies revealed that S. boydii CDPH serotype 18 can survive in bean salad, which contains organic acids and whose pH decreases over time. Acid challenge assays in acidified tryptic soy broth at pH 4.5, acidified Luria-Bertani broth at pH 4.5, and acidified M9 minimal salts medium at pH 2.5 containing amino acids, arginine, or glutamic acid were performed using S. boydii CDPH, S. boydii ATCC 35966, S. flexneri 3136, Escherichia coli O157:H7 dd8872, and E. coli O157:H7 dd642 to compare differences in acid tolerance. Differences in survival of exponential-phase cells were detected in acidified tryptic soy broth and Luria-Bertani broth at pH 4.5. In acidified minimal medium containing arginine, S. boydii strains were able to survive at pH 2.5. The arginine decarboxylase gene (adiA) present in S. boydii is involved in survival at extremely low pH. The discovery of adiA expression in S. boydii serotype 18 by use of an acidified minimal medium challenge and arginine decarboxylase biochemical assay is significant because arginine decarboxylase activity was thought to be unique to E. coli. Sequencing of the rpoS gene from the S. boydii outbreak strain indicates that it is 99% conserved compared with the E. coli K-12 rpoS gene and plays a vital role in survival under acidic conditions.