Crystals of the ferric peroxo (Fe3+-O-O-) species of myoglobin (Mb) have been successfully generated by radiolytic reduction of the oxy Mb crystals at 100 K by irradiation with 1.0 angstrom synchrotron radiation. X-ray diffraction using 0.6 angstrom synchrotron radiation has permitted, for the first time, the determination of the crystal structure of the ferric peroxo heme species, an important intermediate in mono-oxygenase heme enzyme catalysis. The Fe-O-O geometry is similar to that found in the ferrous oxy Mb, the crystal structure of which has been determined also by the 0.6 angstrom radiation. The optical absorption spectra of the crystals affirm that oxy and peroxo species have been maintained during the diffraction measurements, indicating that adverse photoreduction by the incident beam could be mostly avoided by use of the 0.6 angstrom wavelength synchrotron radiation for the diffraction measurements. The Fe-O-O geometries of the oxy and peroxo Mb found in the crystal structures are supported by the QM/MM calculations, which also show that the existence of a strong hydrogen bonding interaction between N epsilon H of His64 and distal oxygen atom in both the oxy and peroxo Mb. The ground state of the peroxo Mb is found as a doublet state, while that of the oxy Mb is an open-shell singlet state with two unpaired alpha and beta electrons mainly distributed on the Fe and O-2 moiety.