Ionophore-siderophore host-guest assemblies composed of 18-crown-6 and ferrioxamine B, benzo-18-crown-6 and ferrioxamine B, and cis-syn-cis-dicyclohexano-18-crown-6 and ferrioxamine B were successfully crystallized, and their structures were determined by single-crystal X-ray diffraction. All three crystal lattices also include solvated Mg(II) and perchlorate ions. The ionophore-siderophore host-guest assembly is noncovalently held together by a hydrogen bonding interaction between the pendant protonated amine in the second coordination sphere of ferrioxamine B and the hydrogen bond acceptor oxygen atoms in the crown ether. The crystals of 18-crown-6:ferrioxamine B host-guest assembly are monoclinic, with space group P2(1)/c, and four molecules per unit cell with dimensions a = 19.8327(11) Angstrom, b = 20.4111 (11) Angstrom, c = 15.1698(8) Angstrom, and beta = 96.435(1)degrees. The crystals of benzo-18-crown-6:ferrioxamine B host-guest assembly are triclinic, with space group P (1) over bar, and two molecules per unit cell with dimensions a = 11.1747(10) Angstrom, b = 16.0580(15) Angstrom, c = 18.4175(17) Angstrom, alpha = 80.469(3) degrees, beta = 81.481(3)degrees and gamma = 70.212(2)degrees. The crystals of cis-syn-cis-dicyclohexano-18-crown-6:ferrioxamine B host-guest assembly are monoclinic, with space group P2(1)/c, and four molecules per unit cell with dimensions a = 20.1473(13) Angstrom, b = 21.5778(15) Angstrom, c = 14.8013(10) Angstrom, and beta = 94.586(2)degrees. The crystal structures of all three host-guest assemblies contain a racemic mixture of Lambda-N-cis, cis and Delta-N-cis, cis coordination isomers of ferrioxamine B. The crystal structures indicate that the steric rigidity of the benzo-18-crown-6 and cis-syn-cis-dicyclohexano-18-crown-6 cavity has a pronounced effect on the conformation of the crown ring and ultimately on the hydrogen bonding interactions between the crown ethers and ferrioxamine B. The structural parameters and the conformational features of the ferrioxamine B guests compare very well with each other and with those of the ferrioxamine B structure obtained in the absence of a host. Structural features relevant to siderophore molecular recognition are discussed.