The genus Guatteria (Annonaceae), an exceptionally rich source of aporphinoid alkaloids, is exclusively neotropical and consists of about 250 species of trees or shrubs indigenous to a region from southern Mexico to southern Brazil. Although these species are relatively widespread, there are few references to their use in traditional medicine. Extracts of the Central American species C. gaumeri have been used in the therapy for gonorrhea, as a diuretic, and as an aid in the expulsion of renal and biliary calculi. An extract of the bark of G. rnodesfa of the Peruvian Amazon may form part of a medicinal thought to possess contraceptive properties. The absence of any literature reference to the constituents of G. diospyroides Baill. coupled with its folkloric medicinal uses prompted an investigation of the alkaloids of this species. G. diospyroides Baill. was collected on December 11, 1989 at Reserva Forestal de San Ramon, Alajuela and identified by Dr. Jorge Gomez Laurito. The air-dried, ground leaves (1.9 kg) were extracted with EtOH and the extract fractionated according to accepted methods. Extensive gradient column chromatography of the basic fraction (CHCl3-MeOH mixtures, silica gel) and preparative TLC (C5H6-Me2CO-NH4OH mixtures, silica gel) afforded the aporphine alkaloids (-)-isopiline (52 mg), (-)-O-methylisopiline (93 mg), and (-)-nornuciferine (57 mg), as well as the oxoaporphine alkaloid O-methylmoschatoline (16 mg). The alkaloids were characterized by direct comparison (UV, IR, 1H-NMR, ElMS, m.p., opt. rotn.) with authentic samples and/or published data.