The ferrichrome-iron receptor of Escherichia coli K-12 encoded by the fhuA gene is a multifunctional outer membrane receptor required for the binding and uptake of ferrichrome and is the receptor for the bacteriophages T5, T1 $ phi$80, and UC-1 as well as for colicin M. The complete nucleotide sequence of the fhuA gene was determined from a 2,902-base-pair fragment of DNA. A single open reading frame was found which translated into a 747-amino acid polypeptide. A signal sequence of 33 amino acids was followed by a mature protein with a molecular weight of 78,992. To identify domains of the protein which are important for FhuA activities, libraries of insertion and deletion mutants were constructed. Export of the mutant FhuA proteins to the outer membrane and receptor functions of the mutant proteins were analyzed. All of the linker insertion mutant proteins and all but three of the deletion mutant FhuA proteins co-fractionated with the outer membrane. Five linker insertion proteins and most deletion proteins were susceptible to cleavage by endogenous proteolytic activity. Some of the mutant receptors conferred wild-type phenotypes: they demonstrated growth promotion by ferrichrome and the same efficiency of plating as that of wild-type FhuA; killing by colicin M was also unaffected. Several mutants were altered in their sensitivities to the lethal agents. The results designated selected regions of the FhuA protein for receptor functions and demonstrate the presence of both shared and unique ligand-responsive domains.