Through bioassay-guided separations of the chemical constituents of the Indonesian medicinal plant Beilschmiedia madang BL. a bisbenzylisoquinoline alkaloid was obtained as the major antimalarial principle. The physicochemical properties of the alkaloid were consistent with the proposed structure of dehatrine. However, the alkaloid isolated by us was shown to be a mixture of two rotational isomers. The X-ray crystallographic analysis of 1 has shown that two rotamers are incorporated in a single crystal in 1:1 ratio. The complex NMR spectrum of 1 has also been defined as a mixture of two rotamers by extensive use of 2D (COSY and COLOC) techniques. Dehatrine has been shown to significantly inhibit the growth of cultured Plasmodium falciparum K1 strain (cholorquine resistant) with similar activity to quinine.