In some recent communications, the isolation and characterization of aporphine and liriodenine type alkaloids substituted at the unusual C-3 position have been described. We now report the isolation of a new benzyltetrahydroisoquinoline alkaloid, thalifendlerine (I), from Thalictrum fendleri. To establish its structure (5,6,7-substituted rather than 6,7,8-substituted), an isomeric base (II) was synthesized and found different from thalifendlerine; treatment with diazomethane gave an O-methyl derivative identical to tetrahydrotakatonine (revised structure IV), confirming thalifendlerine (I) as the first benzylisoquinoline alkaloid substituted at C-5. Additionally, from the quaternary, water-soluble alkaloidal fraction, we isolated known alkaloids (berberine, jatrorrhizine, magnoflorine) and a new optically inactive yellow crystalline alkaloid, thalifendine (V, C₁₉H₁₆O₄N⁺ chloride). Thalifendine's structure was established by reduction to tetrahydrothalifendine (VI), which upon O-methylation gave tetrahydroberberine (VII) identical to an authentic sample; its phenolic function was localized at C-10 by comparison with berberrubine chloride.