Australine 111, a tetrahydroxy pyrrolizidine alkaloid with a unique substitution pattern, has been isolated from seeds of Castanospmnum australe and shown to be a potent and specific inhibitor of amyloglucosidase. The structure was established by a combination of spectroscopic and chemical techniques and confirmed by X-ray crystallography. Australine is the first pyrrolizidine alkaloid to be identified as a glycosidase inhibitor and is the third glucosidase inhibitor, together with the indolizidine alkaloids castanospermine 121 and 6-epicastanospermine [3], to be isolated from C. australe. In addition to these bicyclic alkaloids, the piperidine alkaloid fagomine [SI, a non-inhibitor, has been isolated from the seeds.