The structures of syringostatins A and B produced by Pseudomonas syringae pv. syringae SY12 were determined as 1 and 2, respectively, from NMR and Mass spectrometry. Pseudomonas syringae pv. syringae was a phytopathogenetic bacterium which causes various symptoms to many plants. It is known that many strains of the bacterium produce syringomycin1) and those from citrus give syringotoxin 2) as phytotoxins. Both toxins are believed to be major virulence determinants. The toxins also show wide antimicrobial activity. Although these toxins have been partly characterized, their total structures have not previously been established. 3) We found that P. syringae pv. syringae SY12 isolated in Japan produced phytotoxic antibiotics which were clearly different but closely related to syringomycin and syringotoxin, and we reported preliminarily the isolation and partial characterization of these toxins termed syringostatins. 4) In this communication we describe the structures of the major components, syringostatins A and B. The high-resolution FAB mass spectra indicated that syringostatins A (1) and B (2) contained a chlorine atom and suggested that these molecular formulas were C50H57ClN12O18 and C50H57ClN12O19, respectively. 4) Both 1 and 2 are consisted of one each residue of serine, threonine, ornithine, homoserine, 3-hydroxyaspartic acid, 2-amino-2-butenoic acid (dehydrothreonine) and 2-amino-4-chloro-3-hydroxybutanoic acid (γ-chlorothreonine), and two residues of 2,4-diaminobutanoic acid. 4) The long chain hydroxy acid residue, 3-hydroxytetradecanoic acid, in 1 was replaced by 3,4-dihydroxytetradecanoic acid in 2, this accounting for the difference of the molecular formula between 1 and 2.