Transport of [14C]citrate, ferric [14C]citrate and [55Fe]ferric citrate into Pseudomonas aeruginosa grown in synthetic media containing citrate, succinate, or succinate and citrate as carbon and energy sources was measured. Cells grown in citrate-containing medium transported radiolabelled citrate and iron, whereas the succinate-grown cells transported iron but not citrate. Binding studies revealed that isolated outer and inner membranes of citrate-grown cells contain a citrate receptor, absent from membranes of succinate-grown cells. [55Fe]Ferric citrate bound to the isolated outer membranes of each cell type. The failure of citrate to compete with this binding suggests the presence of a ferric citrate receptor on the outer membranes of each cell type. Citrate induced the synthesis of two outer-membrane proteins of 41 and 19 kDa. A third protein of 17 kDa was more dominant in citrate-grown cells than in succinate-grown cells.