The genes encoding alkaline phosphatase (phoA) and the inducible inorganic phosphate transport system Pst (pstS,C,A,B,U) belong to the PHO regulon. Mutants of Escherichia coli lacking the global regulatory protein integration host factor (MF) show an increased level of alkaline phosphatase and a decreased level of Pst. MF binds weakly but specifically to a DNA fragment containing the promoter region of the pst operon but does not bind to a fragment that includes the promoter region of phoA. It is proposed that MF is a positive regulator of the pst operon and as such controls indirectly the expression of phoA.