A novel peptolide antibiotic, serratiomycin, 1, was isolated from the culture broth of a eubacterium by solvent extraction and column chromatography. The structure of I was determined to be cyclo(D-[~-hydrexydecanoyl-L-leucyi-L-seryI-L-a//othreonyI-D-pbenylalanyl-D-isoleucyl). I exhibited a weak and mostly Gram-positive antibacterial spectrum in vitro.