New cassane diterpenes named neocaesalpins C and D were isolated from the Philippine crude drug calumbibit botanically originating from the seeds of Caesalpinia bonduc (Fabaceae), and their structures were elucidated on the basis of the spectroscopic evidence. These compounds are characterized by the presence of the α, β-butenolide moiety. Although a number of cassane furanoditerpenes have been known to occur in the same plant species, such constituents could not be isolated from the crude drug of Philippine orgin in this study. It is presumed that the chemical difference resulted from chemical differentiation of the species.