A new antibiotic, designated sagamilactam, was isolated from the cultured broth of Actinomadura sp. K13-0306 by Diaion HP-20, silica gel and octadecylsilane column chromatography followed by purification by HPLC. The chemical structure of the new compound was elucidated by spectroscopic analyses, including NMR and MS. The structure of sagamilactam proved to be a new compound consisting of a polyunsaturated and polyoxygenated 34-membered macrocyclic lactam containing diene, triene and tetraene conjugated olefins and a decalin moiety. Sagamilactam showed antitrypanosomal activity with an IC50 value of 0.25±0.11 μM.