Kirrothricin, a new narrow-spectrum antibiotic produced by Streptomyces cinnamoneus strain Tu 89, was isolated in respect to its activity against Clostridium pasteurianum. The molecular formula is C44H64N2O10. Kirrothricin is related to kirromycin, aurodox and other members of this group by its chemical structure, biological activity and the mode of action. Kirromycin(—mocimycin)1・2), a representative of a new chemical class of antibiotics, was described first in 1972. Some other antibiotics, aurodox(=X5108)3,4), efrotomycin5), heneicomycin(=A21A)6), dihydromocimycin7) and probably azdimycin9) were found to be closely related. In the course of our screening for antibiotics active especially against Clostridium pasteurianum, the strain Tu 89 was striking. It was identified as Streptomyces cinnamoneus, which is identical with Streptoverticillum cinnamomeus comb. nov. (A-725)9). Beside aureothin10) and 2-ethyl-5-(3-indolyl)oxazole11) the strain produces a bright yellow compound called kirrothricin21). The latter proved to be a new member of the kirromycin group. In this paper we describe the fermentation, isolation, physicochemical characterization and the biological properties of kirrothricin. The chemical structure is reported in another publication12).