Streptomyces hygroscopicus var. crystallogenes, IFM 1 136 (FERMBP-576), produces antibiotic copiamycin (niphithricin, 2, Fig. I)1 ~4) along with minor macrocyclic lactone antibiotics, neocopiamycin A (3), demalonylcopiamycin (4), demalonylmethylcopiamycin. (5) and guanidolide A, possessing a guanidino alkenyl side chain.5~7) Copiamycin had been found to possess synergistic activity with chlorinated imidazole antimycotics such as clotrimazole, miconazole, econazole and especially ketokoazole.8) Arai et al. had reported that the strain produces a new macrocyclic lactone antibiotic, neocopiamycin B, when cobalt (II) chloride is added into the original medium.9) Neocopiamycin B proved to be more active against some fungi than copiamycin (2) and/or neocopiamycin A (3) and low toxicity for mice (LD50: >25mg/kg, iv).9) In this paper, we describe the skeletal structure of neocopiamycin B.