The structure of a homonojirimycin isomer isolated from Aglaonema treublii and originally proposed as alpha-3,4-di-epi-homonojirimycin was revised to alpha-4-epi-homonojirimycin 3 ("alpha-homoallonojirimycin") on the basis of NMR analysis and synthetic studies. Its activity as a glycosidase inhibitor is compared to that of other homonojirimycin isomers.