Summary: Salmonella typhimurium encounters a variety of acid conditions during both its natural and pathogenic existence. The ability of this organism to respond transcriptionally to low pH is an area of active interest but little knowledge. As part of an ongoing investigation of low-pH adaptation, 18 pH-controlled IacZ operon fusions in Salmonella typhimurium have been identified (15 in this study) and categorized into at least 11 different loci. They include iroA (at 57 min), aciA (99 min), aciB (90-93 min), aciD (ompC, 45 min), aciJ, aciK(33-36 min), aniC (93 min), anil (33-36 min), hyd (59 min), cadA (54 min) and aniG (63 min). All but two were induced by low pH. One of the exceptions, the iron-regulated iroA locus, was induced at high pH. The unusual aciA locus was induced by low pH under semiaerobic conditions but high pH under aerobic conditions. Most of the other aci genes were expressed best under anaerobic conditions. Many of these genes exhibited strict co-inducer requirements for small molecules to be expressed in minimal medium. These included iron for iroA, tyrosine for aniC, I and aciK, mannose for aniG, formate for hyd, lysine for cadA, and unknown components of complex medium for aciA, aciB and aciD. Six regulatory circuits were revealed involving at least five regulatory loci (fur, oxrG, earAB, earC and ompR). As part of the adaptive response to low pH, S. typhimurium will induce an acid protection system called the acid tolerance response (ATR). As has been shown for fur mutations, the oxrG regulatory mutation interfered with the normal induction of this system.