Melleumin A (1), a novel peptide lactone, has been isolated from the laboratory-cultured plasmodium of myxomycete Physarum melleum, and its structure was elucidated by spectral data. Melleumin A (1) consisted of four residues (p-methoxybenzoic acid, L-threonine, glycine, and an unusual amino acid, a tyrosine-attached acetic acid).