In recently reclaimed fields from a mulberry plantation a severe outbreak of zouhibyo (elephant skin-like disease, identical with or similar to russet scab) -like disease was observed on potato tubers cultivated in the spring of 1985 in Kumamoto Prefecture, where potato plants had not been grown for more than twenty years at least. The symptoms most frequently observed were characterized by the presence of shallow, brown, corky reticulations on the tuber surface, which could be roughly divided into three main categories, namely, (A) zouhi (or russet), (B) small reticulations and (C) rough skin, and the combination of two or three of the symptoms, mostly on the tuber. Symptoms (A) predominated and tubers with symptoms (A) and a mixture accunted for more than 95 percent of the affected plants in the varieties May Queen and Dejima cultivated in the spring. Streptomyces spp. were isolated at a higher rate from lesions with (A) and (B) symptoms than (C) . The disease also developed seriously in the autumn potatoes in fields in which crops such as corn or radish as well as potato had been cultivated, in field left fallow during a two year period after the removal of mulberry trees and even in soils collected from a mulberry plantation and a copse in the near vicinity. It was shown that the disease was to soil-and seed tuber-borne, and that temperatures of 20C for 14 hours in the daytime and 18C for 10 hours at night were more conductive to the manifestation of the symptoms than 25 and 30C under the same conditions. The disease was considered to be caused by the agent of zouhibyo, Streptomyces verrucosus SHIBATA et KIMURA sp. nov., which occurred naturally in the mulberry plantation and its vicinity, and whose population increased rapidly under the favorable conditions created by liming and plowing in green manure or compost containing saw dust and livestock litter when the new field were established.