The structure of a new antibiotic designated lydicamycin was elucidated as shown in Fig. 1 by NMR spectral analysis including a variety of 2D techniques. Lydicamycin possesses a novel skeleton containing tetramic acid and amidinopyrrolidine moieties. In the preceding paper, we have described the fermentation, isolation and biological activity of a new antibiotic, lydicamycin, as well as the taxonomy of the producing organism, Streptomyces lydicus 2249-S3. This paper describes the physico-chemical properties and structure elucidation of lydicamycin.