The chemical structures of amicoumacin A, B and C, which are produced by Bacillus pumilus BN-103, were elucidated on the basis of mass, ¹H-NMR and ¹³C-NMR spectral analyses and chemical degradation. Amicoumacin A was derived from amicoumacin B via amicoumacin C. As reported in previous papers, amicoumacins are a complex of closely related isocoumarin compounds. They were separated into three components: amicoumacin A (I, C₂₀H₂₉N₃O₇·HCl), a carboxylic amide with antibacterial, antiinflammatory and antiulcer activities; amicoumacin B (II, C₂₀H₂₅N₂O₅), a carboxylic acid; and amicoumacin C (III, C₂₀H₂₆N₂O₇), a γ-lactone of (II). (II) and (III) showed moderate activity against bacteria but did not exhibit pharmacological activity. The present paper deals with the structural determination and chemical transformation of (I), (II) and (III).