At1g78500, one of the oxidosqualene cyclase (OSC) homologues from Arabidopsis thaliana, was expressed in a lanosterol synthase-deficient yeast strain and the products were analyzed. In addition to the known triterpenes, this OSC was found to produce two new triterpenes, the structures of which were determined by NMR and MS analyses. The new triterpenes are C-ring-seco-beta-amyrin (1) and C-ring-seco-alpha-amyrin (2) and named beta-seco-amyrin and alpha-seco-amyrin, respectively. beta-seco-Amyrin is produced from the oleanyl cation through bond cleavage between C8 and C14, and alpha-seco-amyrin is produced from the ursanyl cation in the same manner. Together with Grob fragmentation catalyzed by another OSC (marneral synthase) from A. thaliana, the formation of seco-amyrins by this OSC revealed that OSCs not only catalyze carbon-carbon bond formations and Wagner-Meerwein rearrangements but also cleave preformed ring systems in cationic intermediates. Based on this information, direct production of other natural seco-triterpenes by OSCs is proposed.