A capillary GC-MS analysis revealed that two stereoisomers of castasterone are contained in immature seeds of Phaseolus vulgaris. 400 MHz proton NMR analysis of the stereoisomers determined they are A ring epimers of castasterone, 3-epicastasterone and 2,3-diepicastasterone. In rice lamina inclination assay, 3-epicastasterone and 2,3-diepicastasterone showed reduced biological activity than that of castasterone. Together with our previous finding that 2-epicastasterone exhibits a less biological activity than CS, this result indicates that epimerization of hydroxyl at C-2 or/and C-3 is/are inactive processes to reduce biological activity of CS after exerting as a bioactive brassinosteroid in P. vulgaris.