The aqueous-alcoholic extract of the bark of GUATTERIA GAUMERI (Mexican Annonaceae) is used for the treatment of cholelithiasis and hypercholesterolemia. Therefore it seems very interesting to perform a phytochemical study of this drug in order to know its composition and to determine, if possible, all the active principles. We describe now the isolation of the main alkaloid named guattegaumerine. This new bisbenzylisoquinoline alkaloid has been assigned structure 1, on the basis of spectroscopic, chemical and chromatographic evidence.