The x-ray crystal structure of ferric Parasponia andersonii hemoglobin has been determined to 2.3 A resolution by molecular replacement with Rice hemoglobin 1 (rHb1). By means of absorbance spectroscopy, electrochemistry and x-ray crystallography, it has been revealed that depending on the oxidation state, the heme iron can exsist in either the pentacoordinate or hexacoordinate form. Pentacoordination of the heme iron is found primarily in the deoxy ferrous (2+) form, whereas the ferric (3+) form is exhibited predominantly in bis-histidine coordination. This is unique among known forms of hemoglobins. Furthermore, the crystallographic data has provided evidence which supports previous works reporting hexacoordination of the heme iron more closely associated with the ferric state (11).