A rare case of pyoderma gangraenosum caused by Rhizopus arrhizus is reported. The patient, a 50-year-old male farmer, was admitted to hospital complaining of gangrening and festering of the right upper arm with severe pain for nearly 2 months. A lesion was found on the inside of right upper arm. The central skin part of the lesion became black dry gangrenous, the periphery was deeply ulcerated with yellow-green pus. The necrotic crust, biceps, triceps and vessels inherited a histopathologically proven fungal infection. The fungus isolated was identified as Rhizopus arrhizus. The infection was successfully treated with itraconazole.