Several fermentable sugars (glucose, lactose, arabinose, and to a lesser extent sucrose) were found to reduce colicin production in a strain of Shigella sonnei P9 (colicins E2 and I) and in a strain of Escherichia coli CA31 (colicin A). Inositol and adonitol, which are apparently not fermented by these colicinogenic strains, did not seem to affect colicin production under our experimental conditions. The results indicated that the production of colicins E2 and A, like the formation of certain R-factor-coded enzymes, may be subject to catabolite repression. A system of mixed cultivation in nutrient broth, of colicinogenic and sensitive strains, was also used; under these conditions, the number of viable units of the sensitive indicator strain decreased more rapidly in the absence than in the presence of added glucose.