The X-ray crystal structure of the formate derivative of ferric loggerhead sea turtle (Caretta caretta) Mb has been determined at 2.0 A resolution (R = 0.164) by difference Fourier techniques. Formate, sitting in the central part of the heme distal site, is coordinated to the heme iron as unidentate ligand, through the O1 oxygen atom, and is hydrogen bonded to the distal His64(E7) NE2 atom through O2. Thermodynamics for formate binding to ferric loggerhead sea turtle Mb, sperm whale Mb, Aplysia limacina Mb, as well as to the VR and VRS mutants of sperm whale Mb were obtained between pH 4.5 and 8.5, at 20.0 degrees C. These results, representing the first structure of a ferric hemoprotein:formate complex solved by X-ray crystallography, outline the role of amino acid residues at positions E7, F8 and E10 in modulating ligand binding properties of oxygen carrying proteins.