Data are presented that establish the structure of a carboxylic acid product of Pseudomonas spp. as 2-[l-oxo-2-cyclopenten-2-ylmethyl] butanoic acid (compound 1). This newly described compound, of probable polyketide origin, may be intimately linked with the biosynthesis of coronafacic acid, and the phytotoxin coronatine. Many pathovars of the plant pathogenic bacterium Pseudomonas syringae are known to produce the phytotoxin coronatine 1. A frequent co-metabolite of coronatine is the polyketide-derived 2 compound coronafacic acid 1 (2), which is a constituent structural part of coronatine. Coronatine biosynthesis involves the ligation of coronafacic acid and 1-amino-l-carboxy-2-ethylcyclopropane 3'4'5. Although coronafacic acid is biosynthesised from one pyruvate and five acetate units, the actual sequence of assembly of the polyketide chain, and mode of cyclizations resulting in the fused ring structure, have not to date been established 6. The structure derived here for 1, a compound often present in culture broths containing coronatine and coronafacic acid, suggests the course of coronafacic acid biosynthesis.