antibiotic, was isolated from the culturebroth of a marine bacterium, Alteromonas rava sp. nov. SANK 73390. Its structure was deduced as a hybrid composed of a pseudomonic acidanalogue and holothin by NMR spectral analysis and chemical degradation. Antimicrobial activity against Gram-positive and Gram-negative bacteria of thiomarinol was stronger than both of pseudomonic acids and pyrrothine antibiotics