Acacia midenii F. Muell., a member of the family Leguminosae, subfamily Mimosoideae, has not previously been examined for the presence of alkaloids. The bark has now been found to contain about 0.6% of the bases N-methyl- and N,N-dimethyl-tryptamine, in the proportions approximately 2 : 3. These have not previously been recognized in Acacia species but have been found in other genera of Leguminosae (Mimosa, Piptadenia, and Lespedeza), in the Apocynaceae (Prestonia), in the Chenopodiaceae (Arthrophytum and Girgensohnia) and in the Gramineae (Phalaris). Since the presence of N,N-dimethyltryptamine in Phalaris tuberosa may be associated with toxicity to stock, the occurrence of these bases in genera such as Acacia which contain species used as forage merits further attention.