Herbimycin, a new antibiotic, was isolated from the fermentation broth of Streptomyces hygroscopicus strain No. AM-3672, a soil isolate. The molecular formula of herbimycin was determined to be C30H42N2O9. Herbimycin was found to have potent herbicidal activity against most mono- and di-cotyledonous plants, especially against Cyperus microiria STEUD. However, Oryza sativa showed strong resistance to herbimycin.