An unusual glucoalkaloid has been crystallized from the title cactus plant. Acid employed in the usual alkaloid extraction schemes decomposed the new nonphenolic alkaloid (pterocereine) into a new phenolic alkaloid (deglucopterocereine) and glucose. β-Glucosidase also hydrolyzed the glucoalkaloid. Spectral analysis of the structures indicated that pterocereine is (―)-l-hydroxymethyl-2-methyl-5-β-0-glycosyl-6, 7-dimethoxy-l, 2, 3, 4-tetrahydroisoquinoline. Further structural conformation was accomplished by carbon-13-hydrogen long-range coupling analysis. Neither compound exhibited significant cytotoxicity in the 9 KB system. © 1979, American Chemical Society. All rights reserved.