The cyclic trihydroxamic acid, N, N', N' '-triacetylfusarinine C, produced by Mycelia sterilia EP-76, is shown to be a ferric ionophore for this organism. The association constant for ferric-N, N', N' '-triacetylfusarinine C complex was determined to be log K=32.5. Other iron chelating agents, such as rhodotorulic acid, citric acid, or the monomeric subunit of triacetylfusarinine C, N-acetyl- fusarinine, delivered iron to the cells by an indirect mechanism involving iron exchange into triacetylfusarinine C. In vitro ferric ion exchange was found to be rapid with triacetylfusarinine C. Gallium uptake rates comparable to those of iron were observed with the chelating agents that transport iron into the cell. Ferrichrome, but not ferrichrome A, was also capable of delivering iron and gallium to this organism, but not by an exchange mechanism. Unlike triacetylfusarinine C, the 14C-ligand of ferrichrome was retained by the cell. A mid-point potential of -690 mV versus the saturated silver chloride electrode was obtained for the ferric-N, N', N' '-triacetylfusarinine C complex, indicating that an unfavorable reduction potential was not the reason for utilizing a hydrolytic mechanism of intracellular iron release from the ferric triacetylfusarinine C chelate. The iron transport system recognizes only the  -cis coordination isomer of ferric-N, N’, N' '-triacetylfusarinine C metal ligand complex even though the -cis configuration predominates in solution. Ferrichrome and ferric-N, N', N' '-triacetylfusarinine C are both transported by the same receptor.