The Australian leguminous tree Castanospermum australe contains the anti-viral glucose analogue indolizidine alkaloid castanospermine. As the result of a search for new bioactive carbohydrate-like compounds we now report the isolation of the novel polyhydroxylated pyrrolizidine alkaloid, 1 epialexine, from the leaves and stems of C. australe. 1-Epialexine is a weak inhibitor of b-mannosidase from Cellullomonas fimi.