A pyridoxal phosphate-dependent tryptophan decarboxylase has been purified 20-fold from seedlings of Phaluris tuberosa. The enzyme activity of the seedlings reached a maximum after 4 days. The enzyme activity is maximal at pH 7.6 and could be demonstrated either by the production of ¹⁴C-tryptamine from methylene ¹⁴C-L-tryptophan or of ¹⁴CO₂ from carboxyl-¹⁴C-L-tryptophan. D-tryptophan was not a substrate but 5-hydroxy-¹⁴C-tryptophan was. The decarboxylation was inhibited at concentrations of 0.1 mM by a number of indole derivatives including N,N-dimethyltryptamine, one of the two major alkaloids produced by the plant. The second alkaloid, 5-methoxy-N,N-dimethyltryptamine did not inhibit at this concentration. A possible role of this enzyme in alkaloid biosynthesis is discussed.