The readily available 2,3:6,7-di-O-isopropylidene-D-glycero-D-gulo-heptono-1,4-lactone [(-)-5] was converted in four synthetic steps into the new iminooctitol (+)-3,7,8-trideoxy-3,7-imino-D-threo-L-galacto-octitol [(+)-10]. Although the piperidine unit of (+)-10 has the absolute configuration of beta-D-galacto-hexopyranosides, this iminoalditol does not inhibit five commercially available beta-galactosidases. However, (+)-10 was found to be a good competitive inhibitor of beta-glucosidases from almond (K-i = 15 mu M) and from Caldocellum saccharolyticum (K-i = 41 mu M).