An antibiotic producing strain of Sh. paradysenteriae Type XI (Boyd III;D1) has been described. When tested against 80 aerobic Enterobacteriaceae the inhibitor was active only against Types I, II, IV, and VIII Sh. paradysenteriae. The susceptibility of one strain of Shigella was no indication that other strains of the same type would be likewise inhibited. The antibiotic was water-soluble, could not be extracted by the common organic solvents and did not pass through cellophane. Observations on the production of the antibiotic under various cultural conditions and attempts to concentrate the active agent by adsorption methods are described.