It is well Known that B. coli. B. dysenteriae and B. cholera etc., so called indole positive Bacillen produce indole from tryptophan. There are many reports of B. coli but only a few reports of B. dysenteriae. Then the author performed experiments with sh. flexneri 2a belonging to B. dysenteriae in order to study the Mechanism of indole production from tryptophan on B. dysenteriae. The results were as follows: 1) Indole production from tryptophan by this organism is the most remarkable between PH 8.0 and 8.5. 2) Tryptophdn is completely converted into indole by this organism and the rate of indole production is almost equal to the rate at which the tryptophan disappears. 3) Of the various divalent metal ions tested, Hg(++), Ni, (++) Co(++) and Cu(++) inhibit strongly indole production by this organism and the most remarkable is the inhibitive action of Hg.(++) 4) Of the various inhibitors tested, KCN inhibits indole production most markedly. 5) When glucose, galactose or mannit are present in the growth medium, indole production is inhibited markedly. In contrast, xylose, lactose, saccharose, sorbit, and glycogen were without influence upon indole production. 6) When the glucose is present in the growth medium, small amount of indole was produced and no appreaciable indole production commenced until all the glucose was completely utilized. The inhibitory effect of glucose is not entirely due to the development of the acid side in culture resulting from the utilization of the glucose by this organism. 7) Tryptophanase activity is markedly reduced when bacterial cells are harvested from a meidum containing glucose.