The title of this paper," Catabolite Repression," is a new name for an old phenomenon, the so-called" glucose effect." The content of this paper should make it clear why the new name has been coined. Briefly stated, the glucose effect is the ability of glucose to inhibit the synthesis of certain enzymes. The phenomenon is easily demonstrated: for example, Aerobacter aerogenes readily forms the inducible enzyme histidase when incubated in a medium which contains a source of nitrogen, such as ammonia, a source of carbon and energy, such as glycerol, and the inducer, histidine. However, when the medium contains glucose, either instead of or in addition to glycerol, the formation of histidase is almost completely inhibited (Magasanik, 1955). The glucose effect was first defined by Epps and Gale (1942) who observed that the formation of amino acid deaminases was inhibited by glucose, and by Monod (1947) who discovered that glucose prevents the formation of enzymes essential for the degradation of other sugars. In both of these instances and in many others, glucose exerts its effect on the formation of inducible enzymes. Nevertheless, inducibility is neither a necessary nor a sufficient condition for the effect. Thus many inducible enzymes, for example penicillinase, nitrate reductase, tetrathionate reductase, and most amino acid decarboxylases, are formed readily in the presence of glucose, while certain constitutive enzymes, such as the invertase of yeast and fi-glucosidase in certain yeast mutants, are not formed in the presence of glucose (Magasanik, 1957). The effect is not entirely specific for glucose. Depending on the organism, compounds closely related to glucose, such as gluconic acid, mannitol, or galacrose, may equal glucose in their inhibitory effects on the formation of glucose-sensitive enzymes. Actually, as will be discussed in detail later, any compound which can serve efficiently as a source of intermediary metabolites, and of energy, may reduce the rate of formation of glucose-sensitive enzymes relative to the rate of formation of other proteins. This can be demonstrafed by examining the rate of protein synthesis and the rate of the induced synthesis of fl-galactosidase in cells of Escherichia cell pregrown in a medium containing glycerol and casein hydrolysate, and then placed in the same glyeerol-casamino acid medium or in a medium from which the glycerol has been omitted. It is found that the omission of glycerol has reduced