Peroxidases are a family of heme-containing enzemes which catalyze the oxidation of a variety of organic and inorganic substrate using hydrogen peroxide. X-ray crystallographic analysis of the peroxidase produced by a fungus Arthromyces ramosus (ARP) showed that the overall folding of ARP is similar to those of other plant-type heme peroxidases. The crystal structure at low pH and spectroscopic study of ARP showed that under physiological conditions the heme of ARP is in the pentacoordinated high-spin state and that at high pH the heme iron is able to bind ammonia, forming the low-spin state. In peroxidases, the position of the water molecule on the distal side of the heme appears to be dominated by the inlidazole ring rather than by interaction with the heme. X-ray analysis of ARP soaked in KI solution revealed that a single iodide ion is located at the entrance of the access channel to the distal side of the heme. The present result is expected to provide a structural basis for electron transfer from the substrate (iodide ion) to oxyfenyl group of the compound I.