The genus Thalictrum is a rich source of benzylisoquinoline-derived alkaloids, with no less than approximately 250 alkaloids having been isolated from some 60 different species. In our continuing investigation of the alkaloids of this genus, we examined an extract of the Chinese medicinal herb Thalictrum petaloideum var. supradecompositum Nakai (Ranunculaceae), which has been used in the treatment of diarrhea, hepatitis, and skin infections but for which there have been no literature reports concerning its constituents. The air-dried whole plant (2.5 kg) was defatted with petrol, extracted with 95% EtOH, and the resulting extract fractionated via accepted methods. Repeated gradient column chromatography (CH₂Cl₂-MeOH mixtures) on silica gel of an alkaloid-rich fraction afforded five alkaloids: the protoberberine berberine, the aporphine (+)-magnoflorine, the isoquinolones noroxyhydrastinine and N-methyl-6,7-dimethoxyisoquinolone, and a new 3,4-dihydroisoquinoline named thalpetaline; the simple phenol tyrosol was also isolated. Thalpetaline iodide was found to be identical (UV, ¹H-NMR, HRMS) to a synthetic sample of 2-methyl-5,6-methylenedioxy-7-methoxy-3,4-dihydroisoquinoline iodide previously prepared in our laboratory, representing the first reported occurrence of this alkaloid in nature. This is also the first reported occurrence of a 3,4-dihydroisoquinoline alkaloid within the genus Thalictrum and the first report of the constituents of T. petaloideum var. supradecompositum.