The structure of a new antibiotic designated lydicamycin was elucidated as shown in Fig. 1 by NMR spectral analysis including a variety of two-dimensional techniques. Lydicamycin was found to possess a novel skeleton containing tetramic acid and amidinopyrrolidine moieties. A novel antibiotic, designated lydicamycin (1), has been isolated from the culture broth of an actinomycete identified as Streptomyces lydicus. The compound was active against Gram-positive bacteria such as Bacillus subrilis (MIC <15 pg/ml) and Staphylococcus aureus (MIC 4.2 pg/ml). We report here the isolation and structure determination of 1 (Fig. 1).