Pentahydroxy-nor-tropane alkaloids, calystegines C₁ and C₂, were isolated from the leaves of greenhouse-cultivated Duboisia leichhardtii, together with calystegines B₁, B₂ and B₄. Calystegine C₁ is a potent inhibitor of β-glucosidase and β-galactosidase, whereas calystegine C₂ exhibited no significant inhibitory activity against these enzymes. Only calystegine C₂, the 2-epimer of calystegine C₁, among calystegines isolated to date exhibited an inhibitory activity against all α-mannosidases tested.