Two new antibiotics, istamycins A and B related to fortimicins1-3) and sporaricins4-6) have been found in the culture filtrate of Streptomyces tenjimariensis nov. sp. which will be described in detail elsewhere. The strain has been isolated from a sea mud sample collected in the shallow sea around Tenjin-island in Sagami-Bay. The isolate is chromogenic, producing light aqua blue aerial mycelium and utilizes only glucose or inositol as the sole source of carbohydrate on PRIDHAM-GOTTLIEB's agar medium. Aerial mycelium is recti-flexibilis and the spore surface is smooth.