<jats:p> <jats:sc>Paigen, Kenneth</jats:sc> (Roswell Park Memorial Institute, Buffalo, N.Y.). Phenomenon of transient repression in <jats:italic>Escherichia coli</jats:italic> . J. Bacteriol. <jats:bold>91:</jats:bold> 1201–1209. 1966.—A family of mutants has been obtained in <jats:italic>Escherichia coli</jats:italic> K-12 in which β-galactosidase is not inducible for approximately one cell generation after the cells are transferred to glucose from other carbon sources. After that period; the enzyme can be induced at the level appropriate to glucose-grown cultures of the parent cells. Among a wide variety of carbon sources, the only one capable of eliciting a state of transient repression is glucose. Conversely, transient repression occurs when cells are transferred to glucose from any of a variety of other carbon sources. The only exceptions to this so far discovered are lactose, gluconate, and xylose. Susceptibility to transient repression in mutants can also be induced in glucose-grown cells by a period of starvation. Mutant cells which have become susceptible to transient repression lose susceptibility in the presence of glucose only when they are under conditions which permit active protein synthesis. The presence of an inducer of β-galactosidase is not required during this time, nor does pre-induction for β-galactosidase diminish the susceptibility of mutants. At least two other catabolite repression-sensitive enzymes (galactokinase and tryptophanase) are also sensitive to transient repression, and the two phenomena are probably related. The absolute specificity of glucose and the pattern of response seen after growth in different carbon sources suggest that the endogenous metabolite which produces these repressions is far more readily derived from glucose in metabolism than it is from any other exogenous carbon source.