Erythrina lithosperma Blume (Leguminosae) is a tall tree, native to the Philippines and Java; it has been naturalized in India in the plains of Bengal. No previous phytochemical work of this species was reported. The alkaloids identified are N-norprotosinomenine (1), protosinomenine (2), erysodienone (3), β-erythroidine, erysopine, erythraline, erythramine, erysodine, erysotrine, erythratine, N,N-dimethyltryptophan, and hypaphorine. Of these, alkaloids (1)-(3) were not previously reported in nature. It was shown by tracer studies that aromatic Erythrina alkaloids are biosynthesized from (+)-N-norprotosinomenine (1) by phenol oxidation to give erysodienone (3). Subsequent transformation (O-methylation, reduction, dehydration, etc.) of the dienone (3) gives rise to aromatic Erythrina alkaloids. The isolation of the two primary building units from E. lithosperma is thus significant from a biogenetic point of view and also complementary to the reported co-occurrence of spiroamine and isoquinoline bases in menispermaceous plants. Recently, Kazuo et al. reported the occurrence of another isoquinoline alkaloid, N-nororientaline (4), in the leaves of Erythrina indica Lam. The existing data in the literature would seem to indicate that N-nororientaline does not lie on the natural biogenetic pathway to the Erythrina alkaloids and is only a by-product which accumulates in the plant.