The effect of pH on chorismate mutase/prephenate dehydratase (chorismate pyruvate mutase/prephenate hydro-lyase (decarboxylating) EC 5.4.99.5/EC 4.2.1.51) from Escherichia coli K12 has been studied. While the maximum velocity of both activities is independent of pH, Km for chorismate or prephenate shows a complex pH dependence. Differences in mutase activity in acetate/phosphate/borate and citrate/phosphate/borate buffers were traced to inhibition by citrate. When a variety of analogues of citrate were tested as possible inhibitors of the enzyme, several were found to inhibit mutase and dehydratase activities to different extents, and by different mechanisms. Thus citrate competitively inhibits mutase activity, but inhibits dehydratase activity by either a non-competitive or an uncompetitive mechanism. Conversely, cis- and trans-aconitate competitively inhibit dehydratase activity, but are partially competitive inhibitors of mutase activity. The differential effects of these inhibitors on the two activities are consistent with the existence of two distinct active sites, but additionally suggest some degree of interconnection between them. The implications of these results for possible mechanisms of catalysis by chorismate mutase/prephenate dehydratase are discussed.