The structures of ebelactone A and B, esterase inhibitors produced by Streptomyces sp. MG7-G1 strain, have been determined to be 3,11-dihydroxy-2,4,6,8,10,12-hexamethyl-9-oxo-6-tetradecenoic 1,3-lactone and 2-ethyl-3,11-dihydroxy-4,6,8,10,12-pentamethyl-9-oxo-6-tetradecenoic 1,3-lactone, respectively, by their spectral analyses.