Biological nitrogen fixation, especially via the legume Rhizobium symbiosis, is important for world agriculture. The productivity of legume crops and pastures is significantly affected by soil acidity; in some cases it is the prokaryotic partner that is pH sensitive. Growth of Rhizobium is adversely affected by low pH, especially in the 'acid stress zone'. Rhizobia exhibit an adaptive acid tolerance response (ATR) that is influenced by calcium concentration. Using Tn5-mutagenesis, gusA fusions and 'proteome' analysis, we have identified a range of genes that are essential for growth at low pH (such as actA, actP, exoR, actR and actS). At least three regulatory systems exist. The two-component sensor-regulator system, actSR, is essential for induction of the adaptive ATR. Two other regulatory circuits exist that are independent of ActR. One system involves the low pH-induced regulator gene, phrR, which may control other low pH-regulated genes. The other circuit, involving a regulator that is yet unidentified, controls the expression of a pH-regulated structural gene (lpiA). We have used pH-responsive gusA fusions to identify acid-inducible genes (such as lpiA), and then attempted to identify the regulators of these genes. The emerging picture is of a relatively complex set of systems that respond to external pH.