Two new alexines have been isolated from Castanospermum australe. The structure of 1,7a-diepialexine was firmly established by X-ray crystallographic analysis of the corresponding 1,7-isopropylidene derivative. The structure of the second new alexine was tentatively assigned as 7,7a-diepialexine or its enantiomer. The abilities of naturally occurring alexines to inhibit mouse gut disaccharidase and fungal glucan 1,4-α-glucosidase are compared. © 1990.