Transvalencin Z was isolated from a culture broth of Nocardia transvalensis IFM 10065, a clinical isolate from a Japanese patient with actinomycotic mycetoma. The transvalencin Z structure was determined using NMR and mass spectrometric analyses. The structure is similar to a partial structure of siderophores such as mycobactins and nocobactins, but the compound has no cytotoxic activity. Transvalencin Z shows a strong antimicrobial activity against Gram-positive bacteria, but shows no activity against Gram-negative bacteria, fungi and tumor cells.