The final steps of jasmonic acid (JA) biosynthesis are thought to involve peroxisomal beta-oxidation, but this has not been directly demonstrated. The last and key step in fatty acid beta-oxidation is catalyzed by 3-ketoacyl-CoA thiolase (KAT) (EC 2.3.1.16). A mutant of Arabidopsis thaliana ecotype Landsberg erecta, which lacks a functional KAT protein and is defective in glyoxysomal fatty acid beta-oxidation has been reported. In this study, the mutant was found to accumulate reduced level of JA in both its wounded cotyledons and leaves, while only the cotyledons accumulate 3-oxo-2-(pent-2'-enyl)-cyclopentane-1-octanoic acid (OPC-8:0). This indicates that a defect in one of the thiolase isoenzymes impairs beta-oxidation of OPC-8:0 to JA. The mutant had sufficient thiolase activity for the synthesis of JA in the unwounded but not in the wounded tissues. Activities of the enzymes in the JA pathway that catalyze the steps, which precede beta-oxidation were not altered by the mutation in a thiolase protein. Thus, reduced levels of JA in the wounded tissues of the mutant were attributed to the defect in a thiolase protein.