Previous studies of Acacia species have led to the isolation of alkaloids of three distinct types. Phenylethylamine, N-methylphenylethylamine, and hordenine occur in many species; tryptamine has been found in many species, and Nb-methyl and NbNb-dimethyl-tryptamines were shown to occur in A. maidenii. A third class of Acacia alkaloid, represented by Na-cinnamoylhistamine, has been isolated from A. argentea and A. polystacha. Examination of the alkaloids from the leaves and stems of Acacia complanata A. Cunn. ex Benth., a species found in southern Queensland, has revealed alkaloids of a further type. The major base, isolated in 0.3% yield, is Nb-methyltetrahydroharman (I; R = CH3). A second alkaloid present in trace amounts only has been identified as tetrahydroharman (I; R = H) by thin-layer chromatography and gas chromatography. Although harman type alkaloids have been isolated from many plant families Nb-methyltetrahydroharman appears to have been isolated previously only from Arthrophytum leptocladum, family Chenopodiaceae. Tetrahydroharman has been found in the Leguminosae in Petalostylis labicheoides, and in trace amounts in Petalostylis labicheoides var. casseoides Benth. No trace of tryptamine or its simple Nb-methyl derivatives could be detected in the alkaloids from A. complanata.