Two mutants are described in which the synthesis of tryptophanase is unusually insensitive to catabolite repression. Neither mutation is linked by transduction to the tryptophanase structural gene, neither mutation renders the synthesis of .beta.-galactosidase insensitive to catabolite repression and the mutations do not permit tryptophanase to be synthesized in strains deficient in adenyl cyclase. During growth in glucose-minimal medium the mutants maintained a similar intracellular concentration of cyclic[c]AMP to their wild-type parent; but since in the wild type the concentration of cAMP was the same in glycerol-minimal medium as in glucose-minimal medium, it is doubtful whether catabolite repression is mediated by measurable changes in the concentration of this nucleotide.