There has been very little experimental work on the tryptophan metabolism of streptomycin-resistant bacteria. The authors performed this experiment in order to find out the differencies of the tryptophan metabolism between streptomycin-sensitive and streptmycin-resistant Shigella flexneri 2a. The results were as follows: 1. When streptomycin-resistant strains were grown in the streptomycin-containing medium, they have quantitatively lower tryptophanase activity than streptomycin-sensitive strains. 2. Streptomycin-resistant strains restores tryptophanase activity when streptomycin is absent in the growth medium, but they are resistant to streptomycin (10,000γ/ml). 3. Tryptophanase activity of both strains is inhibited slightly by divalent metal ions such as Mg, Fe, and Mn, but the inhibitive action of Cu is very remarkable. Natrium azide and arsenite have no effect. Vitamin B(6) and 2.4-dinitrophenol accerelates indole production from tryptophan by this organism. 4. Of the the various antibiotics aureomycin inhibits most markedly tryptophanase activity and chloromycetin does slightly, but streptomycin has no influence. 5. Tryptophanase is an adaptive enzyme, and considered to have essentially no relation to streptomycin-resistant mutation.