Previous studies of the interaction of iron(III) with ferreascidin, a glycoprotein isolated from the blood cells of the stolidobranch ascidian Pyura stolonifera, have shown that the iron’s coordination sphere involves two (3,4-dihydroxyphenyl)alanine (DOPA) residues and possibly one tyrosine (Taylor, S. W.; Hawkins, C. J.; Winter, D. J. Inorg. Chem. 1993, 32, 422). Herein we report variable-field (0-11.1 T) low-temperature Mossbauer spectra which reveal the existence of at least three distinct iron(III)-binding sites. In conjunction with electron paramagnetic resonance spectroscopy of ferreascidin and model complexes, site 1 is found to be a strongly antiferromagnetically coupled (J > 100 cm(-1)) binuclear iron(III) center which may possess a mu-oxo bridging group, while site 2 is also an antiferromagnetically coupled (similar to 20 < J < similar to 80 cm(-1)) binuclear iron(III) center. Site 3 is a linear trinuclear iron cluster in which the three high-spin iron(III) ions are antiferromagnetically coupled with J(12) similar or equal to J(23) > J(13) producing a paramagnetic S = 5/2 ground state. The absence of sulfur and sulfide in ferreascidin indicates that the bridging atoms in these three clusters (sites) must be oxygen and/or nitrogen atoms. Site 3 represents the first example of a trinuclear iron(III) cluster with a paramagnetic S = 5/2 ground state to be found in metalloproteins. Structures for the metal ion binding sites are proposed.