Tomaymycin (2) and oxotomaymycin (3) are antibiotics isolated from Streptomyces achromogenes var. Tomaymycetics. The structure of tomaymycin (2) was determined by degradation reactions together with spectral analysis and by synthesis of the important derivative (8b) containing the basic skeleton of the antibiotic. The structure of oxotomaymycin (3) was also established by chemical correlation to the derivative (7a) of tomaymycin. Through synthesis of derivative (8b) and comparison with the natural product derived from tomaymycin (2), it was confirmed that the ethylidene group is attached to the C-2 position, and the structure of tomaymycin (including the absolute configuration of the C-11a position, same as that of l-hydroxyproline) was established as shown in 2. Oxotomaymycin, isolated from the same fermentation broth, had a molecular formula C₁₅H₁₆O₄N₂ determined by high resolution mass spectral analysis. Its structure was confirmed by conversion to the methyl ether (7a) of tomaymycin, which was identical to the compound derived from tomaymycin (2).